Repent and be baptized
Whether baptism is 'for the forgiveness of sins' as a means or as a sign.
Acts 2:38-39
38Peter said to them, “Repent, and be baptized, every one of you, in the name of Jesus Christ for the forgiveness of sins, and you will receive the gift of the Holy Spirit.
39For the promise is to you, and to your children, and to all who are far off, even as many as the Lord our God will call to himself.”
What each tradition says
Rabbinic / Midrashic
The washing would be acceptable, the source says, if it were used not to put away sins but for the purification of the body. This supposes the soul was thoroughly purified beforehand by righteousness.
Kaufmann Kohler, Samuel Krauss, Baptism (Jewish Encyclopedia, 1901-1906) — On John the Baptist and proselyte immersion (quoting Josephus, 'Ant.' xviii. 5, § 2)
Patristic
Chrysostom says the speaker speaks of the gain: remission of sins and receiving the gift of the Holy Spirit. If one is to receive a gift and if baptism conveys remission, he asks, why delay?
John Chrysostom, Homilies on the Acts of the Apostles, Homily VII (NPNF ser. 1, vol. 11, trans. J. Walker) — Homily VII, on Acts 2:38 ('if baptism conveys remission')
Catholic
Haydock identifies the gift of the Holy Ghost as justifying grace. He says this grace is infused in our hearts by the laver of regeneration.
George Leo Haydock, George Haydock's Catholic Bible Commentary (1859) — Note on Acts 2:38 ('the gift of the Holy Ghost')
Orthodox
Philaret of Moscow states that baptism is a Sacrament. In it, a believer’s body is thrice plunged in water in the name of God the Father, the Son, and the Holy Ghost; he dies to the carnal life of sin and is born again of the Holy Ghost to a life spiritual and holy.
Philaret of Moscow, The Longer Catechism of the Orthodox, Catholic, Eastern Church (trans. R. W. Blackmore), served at pravoslavieto.com — On Baptism, Q. 288 ('What is Baptism?')
Reformed / Protestant
Calvin reads Peter as calling us back unto him by name. Baptism is put in the fourth place, as the seal whereby the promise of grace is confirmed.
John Calvin, Commentary upon the Acts of the Apostles, vol. 1 (ed. Henry Beveridge from the trans. of Christopher Fetherstone; Calvin Translation Society) — Commentary on Acts 2:38 ('for the remission of sins')
Evangelical
Albert Barnes says "for the remission of sins" means not only the sin of crucifying the Messiah but all sins. Baptism itself cannot wash away sin; only God's pardoning mercy through Christ's atonement can. Baptism expresses willingness to be pardoned that way and declares conviction that there is no other way.
Albert Barnes, Notes on the New Testament — Acts (1854 edn.), served at bibliaplus.org — Note on Acts 2:38 ('for the remission of sins')