XX. - Of Preparation For, and Conduct After, the Reception of Baptism.

Preparing for Baptism was a sanctification process that included fasting, repeated prayers (even staying up all night praying), and confessing of our sins to be ready to receive baptism. Prayer and fasting helped subdue the bodies desires and lusts to help fight and overcome temptation. After the baptism it was usually followed by a fast, as the pattern Christ set fasting 40 days after His baptism.

FULL TEACHING

They who are about to enter baptism ought to pray with repeated prayers, fasts, and bendings of the knee, and vigils all the night through, and with the confession of all bygone sins, that they may express the meaning even of the baptism of John: “They were baptized,” saith (the Scripture), “confessing their own sins.” (Mat_3:6) To us it is matter for thankfulness if we do now publicly confess our iniquities or our turpitudes: for we do at the same time both make satisfaction (see de Orat. c. xxiii. ad fin., and the note there.) for our former sins, by mortification of our flesh and spirit, and lay beforehand the foundation of defenses against the temptations which will closely follow. “Watch and pray,” saith (the Lord), “lest ye fall into temptation.” (Mat_26:41) And the reason, I believe, why they were tempted was, that they fell asleep; so that they deserted the Lord when apprehended, and he who continued to stand by Him, and used the sword, even denied Him thrice: for withal the word had gone before, that “no one untempted should attain the celestial kingdoms.” (What reference is referred to is doubtful. The editors point us to Luk_22:28-29; but the reference is unsatisfactory.) The Lord Himself forthwith after baptism temptations surrounded, when in forty days He had kept fast. “Then,” someone will say,” it becomes us, too, rather to fast after baptism.” Well, and who forbids you, unless it be the necessity for joy, and the thanksgiving for salvation? But so far as I, with my poor powers, understand, the Lord figuratively retorted upon Israel the reproach they had cast on the Lord. For the people, after crossing the sea, and being carried about in the desert for forty years, although they were there nourished with divine supplies, nevertheless were more mindful of their belly and their gullet than of God. Thereupon the Lord, driven apart into desert places after baptism, showed, by maintaining a fast of forty days, that the man of God lives “not by bread alone,” but “by the word of God;” (Mat_4:1-4) and that temptations incident to fulness or immoderation of appetite are shattered by abstinence. Therefore, blessed ones, whom the grace of God awaits, when you ascend from that most sacred font of your new birth, and spread your hands for the first time in the house of your mother, together with your brethren, ask from the Father, ask from the Lord, that His own specialties of grace and distributions of gifts (1Co_12:4-12) may be supplied you. “Ask,” saith He, “and ye shall receive.” (Mat_7:7; Luk_11:9: αιτεἰτε,͂ καὶ δοθὴσεται, υμἱν͂ in both places) Well, you have asked, and have received; you have knocked, and it has been opened to you.

~Tertullian- On Baptism Vol. 3