On Baptism by Tertullian
Chapter XIII.--Another Objection: Abraham Pleased God Without Being Baptized Answer Thereto. Old Things Must Give Place to New, and Baptism is Now a Law.
Here, then, those miscreants provoke questions. And so they say, |Baptism is not necessary for them to whom faith is sufficient; for withal, Abraham pleased God by a sacrament of no water, but of faith.| But in all cases it is the later things which have a conclusive force, and the subsequent which prevail over the antecedent. Grant that, in days gone by, there was salvation by means of bare faith, before the passion and resurrection of the Lord. But now that faith has been enlarged, and is become a faith which believes in His nativity, passion, and resurrection, there has been an amplification added to the sacrament, viz., the sealing act of baptism; the clothing, in some sense, of the faith which before was bare, and which cannot exist now without its proper law. For the law of baptizing has been imposed, and the formula prescribed: |Go,| He saith, |teach the nations, baptizing them into the name of the Father, and of the Son, and of the Holy Spirit.| The comparison with this law of that definition, |Unless a man have been reborn of water and Spirit, he shall not enter into the kingdom of the heavens,| has tied faith to the necessity of baptism. Accordingly, all thereafter who became believers used to be baptized. Then it was, too, that Paul, when he believed, was baptized; and this is the meaning of the precept which the Lord had given him when smitten with the plague of loss of sight, saying, |Arise, and enter Damascus; there shall be demonstrated to thee what thou oughtest to do,| to wit -- be baptized, which was the only thing lacking to him. That point excepted, he had sufficiently learnt and believed |the Nazarene| to be |the Lord, the Son of God.|