Extract from : Moral Theology, Bk. 6, nn. 95-7.
Concerning Baptism
Baptism, therefore, coming from a Greek word that means ablution or immersion in water, is distinguished into Baptism of water, of desire and of blood.
We shall speak below of Baptism of water, which was very probably instituted before the passion of Christ the Lord, when Christ was baptised by John. But Baptism of desire is perfect conversion to God by contrition or love of God above all things accompanied by an explicit or implicit desire for true Baptism of water, the place of which it takes as to the remission of guilt, but not as to the impression of the character or as to the removal of all debt of punishment. It is called "of wind" because it takes place by the impulse of the Holy Ghost who is called a wind. Now it is de fide that men are also saved by Baptism of desire, by virtue of the Canon Apostolicam, "de presbutero non baptizato" and of the Council of Trent, session 6, Chapter 4 where it is said that no one can be saved "without the laver of regeneration or the desire for it".
Baptism of blood is the shedding of one's blood, i.e. death, suffered for the Faith or for some other Christian virtue. Now this Baptism is comparable to true Baptism because, like true Baptism, it remits both guilt and punishment as it were ex opere operato. I say as it were because martyrdom does not act by as strict a causality as the sacraments, but by a certain privilege on account of its resemblance to the passion of Christ. Hence martyrdom avails also for infants seeing that the Church venerates the Holy Innocents as true martyrs. That is why Suarez rightly teaches that the opposing view is at least temerarious. In adults, however, acceptance of martyrdom is required, at least habitually from a supernatural motive.
If St. Alphonsus believed that Baptism of Desire was *De Fide*, then does that make him a heretic?
Just a mistaken theological opinion that cannot contradict Church defined Dogma. Theologians are fallible but The Holy Mother Church does not err. Therefore not saint, bishop, or theologian is the binding teaching authority of the Church. Dogmas are truths from Heaven, not to be superseded by theologians, by inverting their authority.
We are taught de fide by the First Vatican Council that the meaning of sacred doctrine can NEVER change and that not even a Pope may teach a new doctrine. Vatican I taught: "
The Holy Ghost was not promised to the successor of Peter that by the revelation of the Holy Ghost they might disclose a new doctrine, but by His help they might guard sacredly the revelation transmitted through the Apostles and the deposit of Faith, and might faithfully set it forth"