It is spiritual cleansing with water.
To be baptized is to one is born again, to receive sanctifying grace, which God’s gift of spiritual life. Jesus revealed the connection between faith and Baptism, when he taught, “He who believes and is baptized will be saved; but he who does not believe will be condemned” (see the Gospel of Mark, 16:16). “Truly, truly, I say to you,” He declares, “unless one is born of water and the Spirit, he cannot enter the kingdom of God” (See John 3:5; emphasis added).
Jesus Himself, though sinless, was baptized by Saint John at the start of His public ministry; see the Gospel of Matthew, 3:13. Note: Jesus was sinless because he was wholly-God and wholly-man. It’s the former, not the latter that is important. Because he was wholly-God his thoughts and actions had to be–by definition one could say–the same as God’s. (He was God, after all.)
So, why would he need to be baptized? He didn’t, but as Saint Ambrose of Milan explained in the fourth century, “The Lord was baptized, not to be cleansed Himself but to cleanse the waters, so that those waters, cleansed by the flesh of Christ which knew no sin, might have the power of Baptism” (Commentary on the Gospel of Luke 2:83).
Before ascending to heaven, Jesus reiterated his message to the Apostles, “Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you” (see Matthew 28:19-20).
A few days later, at Pentecost, Saint Peter is speaking to a crowd. At the end of his sermon, he is asked, “What shall we do?
Peter replies, “Repent, and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins; and you shall receive the gift of the Holy Spirit. For the promise is to you and to your children and to all that are far off, every one whom the Lord our God calls to him” (Acts of the Apostles, 2:38-39).
For the Apostolic Church, Baptism was the gateway to Christian life (see Acts 8:12, 38; 9:18; 10:48). In addition, in Acts 8:37, the Ethiopian eunuch, having received the Gospel from Saint Philip, expresses the desire for Baptism. Likewise, in Acts 16:33, Paul and Silas baptize the Philippian jailer and his entire household “without delay.” Recounting his own conversion, Paul remembers that Ananias had said to him, “And now why do you wait? Rise and be baptized, and wash away your sins, calling on his name” (Acts 22:16). Paul tells the Ephesians that “Christ loved the church and gave himself up for her, that he might sanctify her, having cleansed her by the washing of water with the word” (Paul’s Letter to the Ephesians, 5:25-26; emphasis added). In his Letter to Titus, the Paul wrotes we are saved “by the bath of rebirth.”
The early Christian historical writings affirm rebirth through water Baptism. In about the year 150, for instance, Saint Justin said those who were to be baptized “ are brought by us where there is water, and are reborn in the same manner in which we were ourselves reborn. … For Christ also said, ‘Unless one is born again, he shall not enter into the kingdom of heaven’ (John 3:3)” (First Apology 61).
Around the year 200, Saint Clement of Alexandria wrote, “When we are baptized, we are enlightened. Being enlightened, we are adopted as sons. Adopted as sons, we are made perfect. Made perfect, we are become immortal. … It is a washing by which we are cleansed of sins …” (The Instructor of Children 1:6:26:1, 2). In about 217, Saint Hippolytus of Rome spoke of the advent of Christ to the world “and His manifestation by baptism, and the new birth that was to be to all men, and the regeneration of the laver” (Discourse on the End of the World 1). In about 250, Saint Cyprian of Carthage revealed, “When the stain of my past life had been washed away by means of the water of rebirth, a light from above poured itself upon my chastened and now pure heart; afterwards through the Spirit which is breathed from heaven, a second birth made of me a new man” (Letter to Donatus 4).
Note that the Sacrament of Baptism is prefigured throughout the Old Testament. In Genesis 1:2, it is written that at Creation, “The Spirit of God … moving over the face of the waters,” and the ancient flood that purified the earth are both baptismal metaphors. As Saint Peter wrote, “In the days of Noah, during the building of the ark … a few, that is, eight persons, were saved through water. Baptism, which corresponds to this, now saves you, not as a removal of dirt from the body but as an appeal to God for a clear conscience, through the resurrection of Jesus Christ” (see Peter’s first letter, 3:20-21; emphasis added).
The Prophet Elisha’s instruction to the Syrian general Naaman, who had come to him seeking a cure for his leprosy, points to baptismal rebirth. “Go and wash in the Jordan seven times,” the prophet tells him, “and your flesh shall be restored, and you shall be clean” (see the Second Book of Kings 5:10 and Leviticus, 14:7). As he writes in his First Letter to the Corinthians (10:2), Saint Paul sees figures of Baptism in the cloud of fire and smoke that accompanied the Israelites through the wilderness and in the waters of the Red Sea through which they passed. (He also speaks of the Old-Covenant rite of circumcision as the precursor of Baptism in his Letter to the Colossians (2:11-12).