http://www.newadvent.org/cathen/02258b.htm#VI

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Necessity of baptism
Theologians distinguish a twofold necessity, which they call a necessity of means (medii) and a necessity of precept (præcepti). The first (medii) indicates a thing to be so necessary that, if lacking (though inculpably), salvation can not be attained. The second (præcepti) is had when a thing is indeed so necessary that it may not be omitted voluntarily without sin; yet, ignorance of the precept or inability to fulfill it, excuses one from its observance.

Baptism is held to be necessary both necessitate medii and præcepti. This doctrine is rounded on the words of Christ. In John 3, He declares: "Unless a man be born again of water and the Holy Ghost, he can not enter into the kingdom of God." Christ makes no exception to this law and it is therefore general in its application, embracing both adults and infants. It is consequently not merely a necessity of precept but also a necessity of means.

This is the sense in which it has always been understood by the Church, and the Council of Trent (Sess, IV, cap, vi) teaches that justification can not be obtained, since the promulgation of the Gospel, without the washing of regeneration or the desire thereof (in voto). In the seventh session, it declares (can. v) anathema upon anyone who says that baptism is not necessary for salvation. We have rendered votum by "desire" for want of a better word. The council does not mean by votum a simple desire of receiving baptism or even a resolution to do so. It means by votum an act of perfect charity or contrition, including, at least implicitly, the will to do all things necessary for salvation and thus especially to receive baptism.

The absolute necessity of this sacrament is often insisted on by the Fathers of the Church, especially when they speak of infant baptism. Thus St. Irenæus (Against Heresies 2.22): "Christ came to save all who are reborn through Him to God — infants, children, and youths" (infantes et parvulos et pueros). St. Augustine (On the Soul, Book III) says "If you wish to be a Catholic, do not believe, nor say, nor teach, that infants who die before baptism can obtain the remission of original sin." A still stronger passage from the same doctor (Epistle 28) reads:"Whoever says that even infants are vivified in Christ when they depart this life without the participation of His Sacrament (Baptism), both opposes the Apostolic preaching and condemns the whole Church which hastens to baptize infants, because it unhesitatingly believes that otherwise they can not possibly be vivified in Christ," St. Ambrose (II De Abraham., c. xi) speaking of the necessity of baptism, says:" No one is excepted, not the infant, not the one hindered by any necessity."

In the Pelagian controversy we find similarly strong pronouncements on the part of the Councils of Carthage and Milevis, and of Pope Innocent I. It is owing to the Church's belief in this necessity of baptism as a means to salvation that, as was already noted by St. Augustine, she committed the power of baptism in certain contingencies even to laymen and women. When it is said that baptism is also necessary, by the necessity of precept (praecepti), it is of course understood that this applies only to such as are capable of receiving a precept, viz. adults.

The necessity in this case is shown by the command of Christ to His Apostles (Matthew 28): "Go and teach all nations, baptizing them", etc. Since the Apostles are commanded to baptize, the nations are commanded to receive baptism. The necessity of baptism has been called in question by some of the Reformers or their immediate forerunners. It was denied by Wyclif, Bucer, and Zwingli. According to Calvin it is necessary for adults as a precept but not as a means. Hence he contends that the infants of believing parents are sanctified in the womb and thus freed from original sin without baptism. The Socinians teach that baptism is merely an external profession of the Christian faith and a rite which each one is free to receive or neglect.

An argument against the absolute necessity of baptism has been sought in the text of Scripture: "Unless you eat the flesh of the Son of man and drink his blood, you shall not have life in you" (John 6). Here, they say, is a parallel to the text: "Unless a man be born again of water". Yet everyone admits that the Eucharist is not necessary as a means but only as a precept. The reply to this is obvious. In the first instance, Christ addresses His words in the second person to adults; in the second, He speaks in the third person and without any distinction whatever.

Another favorite text is that of St. Paul (1 Corinthians 7): "The unbelieving husband is sanctified by the believing wife; and the unbelieving wife is sanctified by the believing husband; otherwise your children should be unclean; but now they are holy." Unfortunately for the strength of this argument, the context shows that the Apostle in this passage is not treating of regenerating or sanctifying grace at all, but answering certain questions proposed to him by the Corinthians concerning the validity of marriages between heathens and believers. The validity of such marriages is proved from the fact that children born of them are legitimate, not spurious. As far as the term "sanctified" is concerned, it can, at most, mean that the believing husband or wife may convert the unbelieving party and thus become an occasion of their sanctification.

A certain statement in the funeral oration of St. Ambrose over the Emperor Valentinian II has been brought forward as a proof that the Church offered sacrifices and prayers for catechumens who died before baptism. There is not a vestige of such a custom to be found anywhere. St. Ambrose may have done so for the soul of the catechumen Valentinian, but this would be a solitary instance, and it was done apparently because he believed that the emperor had had the baptism of desire. The practice of the Church is more correctly shown in the canon (xvii) of the Second Council of Braga: "Neither the commemoration of Sacrifice [oblationis] nor the service of chanting [psallendi] is to be employed for catechumens who have died without the redemption of baptism." The arguments for a contrary usage sought in the Second Council of Arles (c. xii) and the Fourth Council of Carthage (c. lxxix) are not to the point, for these councils speak, not of catechumens, but of penitents who had died suddenly before their expiation was completed. It is true that some Catholic writers (as Cajetan, Durandus, Biel, Gerson, Toletus, Klee) have held that infants may be saved by an act of desire on the part of their parents, which is applied to them by some external sign, such as prayer or the invocation of the Holy Trinity; but Pius V, by expunging this opinion, as expressed by Cajetan, from that author's commentary on St. Thomas, manifested his judgment that such a theory was not agreeable to the Church's belief.

Substitutes for the sacrament
The Fathers and theologians frequently divide baptism into three kinds: the baptism of water (aquæ or fluminis), the baptism of desire (flaminis), and the baptism of blood (sanguinis). However, only the first is a real sacrament. The latter two are denominated baptism only analogically, inasmuch as they supply the principal effect of baptism, namely, the grace which remits sins. It is the teaching of the Catholic Church that when the baptism of water becomes a physical or moral impossibility, eternal life may be obtained by the baptism of desire or the baptism of blood.

The baptism of desire
The baptism of desire (baptismus flaminis) is a perfect contrition of heart, and every act of perfect charity or pure love of God which contains, at least implicitly, a desire (votum) of baptism. The Latin word flamen is used because Flamen is a name for the Holy Ghost, Whose special office it is to move the heart to love God and to conceive penitence for sin. The "baptism of the Holy Ghost" is a term employed in the third century by the anonymous author of the book "De Rebaptismate". The efficacy of this baptism of desire to supply the place of the baptism of water, as to its principal effect, is proved from the words of Christ. After He had declared the necessity of baptism (John 3), He promised justifying grace for acts of charity or perfect contrition (John 14): "He that loveth Me, shall be loved of my Father: and I will love him and will manifest myself to him." And again: "If any one love me, he will keep my word, and my Father will love him, and we will come to him, and will make our abode with him." Since these texts declare that justifying grace is bestowed on account of acts of perfect charity or contrition, it is evident that these acts supply the place of baptism as to its principal effect, the remission of sins. This doctrine is set forth clearly by the Council of Trent. In the fourteenth session (cap. iv) the council teaches that contrition is sometimes perfected by charity, and reconciles man to God, before the Sacrament of Penance is received. In the fourth chapter of the sixth session, in speaking of the necessity of baptism, it says that men can not obtain original justice "except by the washing of regeneration or its desire" (voto). The same doctrine is taught by Pope Innocent III (cap. Debitum, iv, De Bapt.), and the contrary propositions are condemned by Popes Pius V and Gregory XII, in proscribing the 31st and 33rd propositions of Baius.

We have already alluded to the funeral oration pronounced by St. Ambrose over the Emperor Valentinian II, a catechumen. The doctrine of the baptism of desire is here clearly set forth. St. Ambrose asks: "Did he not obtain the grace which he desired? Did he not obtain what he asked for? Certainly he obtained it because he asked for it." St. Augustine (On Baptism, Against the Donatists, IV.22) and St. Bernard (Ep. lxxvii, ad H. de S. Victore) likewise discourse in the same sense concerning the baptism of desire. If it be said that this doctrine contradicts the universal law of baptism made by Christ (John 3), the answer is that the lawgiver has made an exception (John 14) in favor of those who have the baptism of desire. Neither would it be a consequence of this doctrine that a person justified by the baptism of desire would thereby be dispensed from seeking after the baptism of water when the latter became a possibility. For, as has already been explained the baptismus flaminis contains the votum of receiving the baptismus aquæ. It is true that some of the Fathers of the Church arraign severely those who content themselves with the desire of receiving the sacrament of regeneration, but they are speaking of catechumens who of their own accord delay the reception of baptism from unpraiseworthy motives. Finally, it is to be noted that only adults are capable of receiving the baptism of desire.

The baptism of blood
The baptism of blood (baptismus sanquinis) is the obtaining of the grace of justification by suffering martyrdom for the faith of Christ. The term "washing of blood" (lavacrum sanguinis) is used by Tertullian (On Baptism 16) to distinguish this species of regeneration from the "washing of water" (lavacrum aquæ). "We have a second washing", he says "which is one and the same [with the first], namely the washing of blood." St. Cyprian (Epistle 73) speaks of "the most glorious and greatest baptism of blood" (sanguinis baptismus). St. Augustine (City of God 13.7) says: "When any die for the confession of Christ without having received the washing of regeneration, it avails as much for the remission of their sins as if they had been washed in the sacred font of baptism."

The Church grounds her belief in the efficacy of the baptism of blood on the fact that Christ makes a general statement of the saving power of martyrdom in the tenth chapter of St. Matthew: "Every one therefore that shall confess me before men, I will also confess him before my Father who is in heaven" (verse 32); and: "He that shall lose his life for me shall find it" (verse 39). It is pointed out that these texts are so broadly worded as to include even infants, especially the latter text. That the former text also applies to them, has been constantly maintained by the Fathers, who declare that if infants can not confess Christ with the mouth, they can by act. Tertullian (Against the Valentinians 2) speaks of the infants slaughtered by Herod as martyrs, and this has been the constant teaching of the Church.

Another evidence of the mind of the Church as to the efficacy of the baptism of blood is found in the fact that she never prays for martyrs. Her opinion is well voiced by St. Augustine (Tractate 74 on the Gospel of John): "He does an injury to a martyr who prays for him." This shows that martyrdom is believed to remit all sin and all punishment due to sin. Later theologians commonly maintain that the baptism of blood justifies adult martyrs independently of an act of charity or perfect contrition, and, as it were, ex opere operato, though, of course, they must have attrition for past sins. The reason is that if perfect charity, or contrition, were required in martyrdom, the distinction between the baptism of blood and the baptism of desire would be a useless one. Moreover, as it must be conceded that infant martyrs are justified without an act of charity, of which they are incapable, there is no solid reason for denying the same privilege to adults. (Cf. Francisco Suárez, De Bapt., disp. xxxix.)