Baptism in water, as a divine ordinance connected with repentance and forgiveness of sins, was already known to Israel through John the Baptist; he announced it as a preparation for the coming Messiah, Who should baptise in the Holy Ghost, and also open the Kingdom of Heaven. (Mt. 3:1-12, Mk. 1:1-8, Lk. 3:1-17, John 1:26, 27, 33.)
Christian Baptism, however, had an even deeper meaning added to it after the death and resurrection of Christ.
(a) It is the way appointed by God whereby we wash away in water the sins which God has forgiven through the blood of Jesus. and receive a glorious consciousness of complete cleansing from the past. (Eph. 1:7, Rev. 7:14, Acts 22:16, I Peter 3:21, Mk. 16:16, Titus 3:5, Rev. 1:5 (R.V.), Heb. 10:22, Ps. 103:12.)
(b) It is also an act which typifies the death and burial of the old worldly unbeliever and the rising up of a new spiritual believer to live in a new and holy fellowship. i.e.. that of the Father and the Son and the Holy Spirit. It is the birth of a sanctification which is to grow to perfection. (Mt. 28:19, Rom. 6:1-11, 1:17, Col. 2:12, 3:1-4, I John 1:1-3, II Cor. 7:1, 3:18, Heb. 12:6-14, John 1:16.)
N.B.4. Christian Baptism, being in the Name of the Spirit, as well as of the Father and the Son, must include some instruction about the Spirit of God, and so paves the way for the next Foundation Truth, the Baptism in the Spirit. (Mt. 28:19.)