St. Augustine, Church Father and Doctor of the Church (4th-5th Century): The Seven Books of Augustin, Bishop of Hippo, On Baptism, Against the Donatists, Book IV, Ch 22: "That the place of baptism is sometimes supplied by martyrdom is supported by an argument by no means trivial, which the blessed Cyprian adduces from the thief, to whom, though he was not baptized, it was yet said, "To-day shall thou be with me in Paradise." On considering which, again and again, I find that not only martyrdom for the sake of Christ may supply what was wanting of baptism, but also faith and conversion of heart, if recourse may not be had to the celebration of the mystery of baptism for want of time. For neither was that thief crucified for the name of Christ, but as the reward of his own deeds; nor did he suffer because he believed, but he believed while suffering. It was shown, therefore, in the case of that thief, how great is the power even without the visible sacrament of baptism, of what the apostle says, "With the heart man believeth unto righteousness, and with the mouth confession is made unto salvation." But the want is supplied invisibly only when the administration of baptism is prevented, not by contempt for religion, but by the necessity of the moment."
Ch23: "But as in the thief, to whom the material administration of the sacrament was necessarily wanting, the salvation was complete, because it was spiritually present through his piety, so, when the sacrament itself is present, salvation is complete, if what the thief possessed be unavoidably wanting."
Ch24: "And as in the thief the gracious goodness of the Almighty supplied what had been wanting in the sacrament of baptism, because it had been missing not from pride or contempt, but from want of opportunity..."
Ch25: "By all these considerations it is proved that the sacrament of baptism is one thing, the conversion of the heart another; but that man's salvation is made complete through the two together. Nor are we to suppose that, if one of these be wanting, it necessarily follows that the other is wanting also; because the sacrament may exist in the infant without the conversion of the heart; and this was found to be possible without the sacrament in the case of the thief, God in either case filling up what was involuntarily wanting. But when either of these requisites is wanting intentionally, then the man is responsible for the omission. And baptism may exist when the conversion of the heart is wanting; but, with respect to such conversion, it may indeed be found when baptism has not been received, but never when it has been despised."
From City of God, Book XIII, Chapter 7: "Of the Death Which the Unbaptized Suffer for the Confession of Christ: For whatever unbaptized persons die confessing Christ, this confession is of the same efficacy for the remission of sins as if they were washed in the sacred font of baptism. For He who said, "Unless a man be born of water and of the Spirit, he cannot enter into the kingdom of God," John 3:5 made also an exception in their favor, in that other sentence where He no less absolutely said, "Whosoever shall confess me before men, him will I confess also before my Father which is in heaven;" Matthew 10:32 and in another place, "Whosoever will lose his life for my sake, shall find it." Matthew 16:25"
A Treatise on the Soul and Its Origin, Book II, Ch17, Disobedient Compassion and Compassionate Disobedience Reprobated and Martyrdom In Lieu Of Baptism: "Truth, by the mouth of Itself incarnate, proclaims as if in a voice of thunder: "Except a man be born again of water and of the Spirit, he cannot enter into the kingdom of God." And in order to except martyrs from this sentence, to whose lot it has fallen to be slain for the name of Christ before being washed in the baptism of Christ, He says in another passage, "He that loseth his life for my sake shall find it."
A Treatise On the Soul and Its Origin, by Aurelius Augustin, Bishop of Hippo; In Four Books, 419, Book 1, CH 11, Title Of Chapter 11: "Martyrdom for Christ Supplies the Place of Baptism. The Faith of the Thief Who Was Crucified Along with Christ Taken As Martyrdom And Hence for Baptism".
On the Soul and Its Origin, Book 1, Ch 10: "Moreover, from the time when He said, "Except a man be born of water and of the Spirit, he cannot enter into the kingdom of heaven;" and again, "He that loseth his life for my sake shall find it; " no one becomes a member of Christ except it be either by baptism in Christ, or death for Christ."