Question 72: Is then the external baptism with water the washing away of sin itself?

Answer: Not at all: (a) for the blood of Jesus Christ only, and the Holy Ghost cleanse us from all sin. (b)

(a) Matt.3:11 I indeed baptize you with water unto repentance: but he that cometh after me is mightier than I, whose shoes I am not worthy to bear: he shall baptize you with the Holy Ghost, and with fire:
1 Pet.3:21 The like figure whereunto even baptism doth also now save us (not the putting away of the filth of the flesh, but the answer of a good conscience toward God,) by the resurrection of Jesus Christ:
Eph.5:26 That he might sanctify and cleanse it with the washing of water by the word,
Eph.5:27 That he might present it to himself a glorious church, not having spot, or wrinkle, or any such thing; but that it should be holy and without blemish.
(b) 1 John 1:7 But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin.
1 Cor.6:11 And such were some of you: but ye are washed, but ye are sanctified, but ye are justified in the name of the Lord Jesus, and by the Spirit of our God.