Catechism of the. Council of Trent, McHugh and Callan, 1923
Baptism Of Infants Should Not Be Delayed
The faithful are earnestly to be exhorted to take care that their children be brought to the church, as soon as it can be done with safety, to receive solemn Baptism. Since infant children have no other means of salvation except Baptism, we may easily understand how grievously those persons sin who permit them to remain without the grace of the Sacrament longer than necessity may require, particularly at an age so tender as to be exposed to numberless dangers of death.
Ordinarily They Are Not Baptised At Once
On adults, however, the Church has not been accustomed to confer the Sacrament of Baptism at once, but has ordained that it be deferred for a certain time. The delay is not attended with the same danger as in the case of infants, which we have already mentioned; should any unforeseen accident make it impossible for adults to be washed in the salutary waters, their intention and determination to receive Baptism and their repentance for past sins, will avail them to grace and righteousness.
Nay, this delay seems to be attended with some advantages. And first, since the Church must take particular care that none approach this Sacrament through hypocrisy and dissimulation, the intentions of such as seek Baptism, are better examined and ascertained. Hence it is that we read in the decrees of ancient Councils that Jєωιѕн converts to the Catholic faith, before admission to Baptism, should spend some months in the ranks of the catechumens.
Furthermore, the candidate for Baptism is thus better instructed in the doctrine of the faith which he is to profess, and in the practices of the Christian life. Finally, when Baptism is administered to adults with solemn ceremonies on the appointed days of Easter and Pentecost only greater religious reverence is shown to the Sacrament.
Mgr. J. H. Hervé, Manuale Theologiae Dogmaticae (Vol. III: chap. IV) - 1931
II. On those for whom Baptism of water can be supplied:
"The various baptisms: from the Council of Trent itself and from the things stated, it stands firm that Baptism is necessary, yet in fact or in desire; therefore in an extraordinary case it can be supplied. Further, according to the Catholic doctrine, there are two things by which the sacrament of Baptism can be supplied, namely an act of perfect charity with the desire of Baptism and the death as martyr. Since these two are a compensation for Baptism of water, they themselves are called Baptism, too, in order that they may be comprehended with it under one as it were generic name; so the act of love with desire for Baptism is called Baptismus flaminis (Baptism of the Spirit) and the martyrium (Baptism of Blood)."
Pope Pius XII, Address to Italian Midwives
If what We have said up to now deals with the protection and the care of natural life, it should hold all the more in regard to the supernatural life which the newly born infant receives with Baptism. In the present economy there is no other way of communicating this life to the child who has not yet the use of reason. But, nevertheless, the state of grace at the moment of death is absolutely necessary for salvation. Without it, it is not possible to attain supernatural happiness, the beatific vision of God. An act of love can suffice for an adult to obtain sanctifying grace and supply for the absence of Baptism; for the unborn child or for the newly born, this way is not open.