1. In every New Testament command and instance of baptism repentance and faith precede baptism
Acts 2:37-38, 41
37 Now when they heard this, they were pierced to the heart, and said to Peter and the rest of the apostles, "Brethren, what shall we do?" 38 And Peter said to them, "Repent, and let each of you be baptized in the name of Jesus Christ for the forgiveness of your sins; and you shall receive the gift of the Holy Spirit. . ." 41 So then, those who had received his word were baptized; and there were added that day about three thousand souls.
2. There are no instances of infant baptism in the Bible.
What about household baptisms (Acts 16:15, 33; 1 Corinthians 1:16)?
It is an argument from silence that infants were included in these three occasions. Moreover, in Acts 16:30-33 Luke points that the Word of God was spoken to all those who were baptized, thus suggesting that not infants, but those who could hear the Word, were baptized.
30 [The jailer said], "Sirs, what must I do to be saved?" 31 And they said, "Believe in the Lord Jesus, and you shall be saved, you and your household." 32 And they spoke the word of the Lord to him together with all who were in his house. 33 And he took them that very hour of the night and washed their wounds, and immediately he was baptized, he and all his household. 34 And he brought them into his house and set food before them, and rejoiced greatly, having believed in God with his whole household.
3. Baptism IS described by Paul as an expression of faith.
11 and in Him you were also circumcised with a circumcision made without hands, in the removal of the body of the flesh by the circumcision of Christ; 12 having been buried with Him in baptism, in which [i.e., baptism] you were also raised up with Him through faith in the working of God, who raised Him from the dead.
Thus baptism is an expression of faith, and the raising with Christ that happens in baptism happens by virtue of baptism's being an expression of faith - which infants cannot perform.
4. Baptism is described by Peter as an appeal to God by the person being baptized.
1 Peter 3:18-21
18 For Christ also died for sins once for all, the just for the unjust, in order that He might bring us to God, having been put to death in the flesh, but made alive in the spirit; 19 in which also He went and made proclamation to the spirits now in prison, 20 who once were disobedient, when the patience of God kept waiting in the days of Noah, during the construction of the ark, in which a few, that is, eight persons, were brought safely through the water. 21 And corresponding to that, baptism now saves you - not the removal of dirt from the flesh, but an appeal to God for a good conscience - through the resurrection of Jesus Christ,
Baptism saves in the sense that it is the outward expression of an inward appeal to God, not as a mere water ritual. It saves the way the confession of the lips saves in Romans 10:9 - insofar as the confession of the lips is an expression of the faith of the heart.
But what about the sign of the covenant made with the children of Israelites in the Old Covenant?
7 "And I will establish My covenant between Me and you and your descendants after you throughout their generations for an everlasting covenant, to be God to you and to your descendants after you. 8 And I will give to you and to your descendants after you, the land of your sojournings, all the land of Canaan, for an everlasting possession; and I will be their God." 9 God said further to Abraham, "Now as for you, you shall keep My covenant, you and your descendants after you throughout their generations. 10 This is My covenant, which you shall keep, between Me and you and your descendants after you: every male among you shall be circumcised."
[Infants of Christian parents] belong to the covenant and people of God . . . they also are to be baptized as a sign of the covenant, to be ingrafted into the Christian church and distinguished from the children of unbelievers, as was done in the Old Testament by circumcision, in place of which in the New Testament baptism is appointed.
Westminster Directory for the Public Worship of God
The seed and posterity of the faithful born within the church have by their birth an interest in the covenant and right to the seal of it and to the outward privileges of the church under the gospel, no less than the children of Abraham in the time of the Old Testament . . .
Why is baptism not administered to the children of Christian parents in the New Covenant as circumcision was administered to the children of Jewish parents in the former covenant?
5. Because the New Covenant members are not defined by physical descent, as the old covenant members were, but by God's writing his law on their heart and calling them to himself and bringing them to repentance and faith.
In accord with this narrowing of the covenant people to those who are truly born of God, the new sign of the covenant is meant to signify that a person is indeed part of that new born covenant community, which is evident by faith.
In the same way that a change in the sign came in to allow both men and women to participate in the sign (baptism instead of circumcision), thus making it clearer than before that women and men are equal heirs of salvation (1 Peter 3:7), so also a change in the recipients of the sign came in to make it clearer that under the New Covenant the people of God are not determined at all by physical descent, but by spiritual transformation, evidenced in faith.
5.1 John the Baptist called for baptism for those already having the sign of the covenant, showing that a new meaning was being given to the sign - no longer pointing to physical descent from Abraham, but rather spiritual descent through faith and repentance.
7 But when he saw many of the Pharisees and Sadducees coming for baptism, he said to them, "You brood of vipers, who warned you to flee from the wrath to come? 8 Therefore bring forth fruit in keeping with repentance; 9 and do not suppose that you can say to yourselves, 'We have Abraham for our father'; for I say to you, that God is able from these stones to raise up children to Abraham."
5.2 Jesus affirmed John's ministry and defined the children of God not as those born of certain parents but those born of God through faith.
12 But as many as received Him, to them He gave the right to become children of God, even to those who believe in His name, 13 who were born not of blood, nor of the will of the flesh, nor of the will of man, but of God.
5.3 Paul clarified that the children of Abraham to whom the promise was made were not those born according to the flesh, but those born according to promise. Children of promise and children of the flesh are not the same.
6 But it is not as though the word of God has failed. For they are not all Israel who are descended from Israel; 7 neither are they all children because they are Abraham's descendants, but: "through Isaac your descendants will be named." 8 That is, it is not the children of the flesh who are children of God, but the children of the promise are regarded as descendants.
6 Even so Abraham believed God, and it was reckoned to him as righteousness. 7 Therefore, be sure that it is those who are of faith who are sons of Abraham.
5.4. The children to whom the promise is made are the children who are "called," and the call of God is free and bound to no physical family.
For the promise is for you and your children, and for all who are afar off, as many as the Lord our God shall call to Himself.