The English word “baptism” comes from the Greek verb baptizo which means to dip or immerse. The word was used in a variety of ways such as immersing something in water, washing something or dying fabric. In other words, baptizo was a pretty ordinary word and did not inherently have any of the theological or ecclesial overtones it has today. John being called the Baptist did not mean he was a member of the Southern Baptist Convention neither does it refer to what happens in a Roman Catholic setting where a priest pours water over an infant initiating them into the church. (Some modern translators prefer to call him John the Immerser to avoid all the baggage associated with the word!) Now to sidestep the accrued meanings we have to go back to 1st century Roman Palestine to understand how the word was used in the New Testament in its original context.The particular forms of baptizo from which we get baptism, that is baptisma and the the more common form is bapstismos, occur only within the New Testament and not anywhere else in Greek literature. However, in whatever form the word appeared in Greek literature, it usually had a cultic or religious meaning. Now the Greeks had sacred pools and so did the Jews so baptism, as it was used in the NT, meant a ritual washing. The aim was not to wash away dirt or remove germs (which they had no concept of anyway) but to make something ritually pure. For example in Hebrews it says,
Therefore let us leave the elementary doctrine of Christ and go on to maturity, not laying again a foundation of repentance from dead works and of faith toward God, and of instruction about washings (baptismos), the laying on of hands, the resurrection of the dead, and eternal judgment. – Hebrew 6:1-2 ESV
Here, instead of going with the older convention that appears in the King James Version for example, they rightly translate it as washing, as in ritual washing. Now there were different types of ritual washings in that world. The most significant type for the New Testament is the one the prophet John performs which lends him the famous moniker “The Baptist.” It is significant that the first instance of ritual cleansing we are introduced to in the New Testament narrative takes on a significant new symbolic meaning in their world. Dr. Craig Keener in his bestselling New Testament Background Commentary writes,
Of the many kinds of ceremonial washings in Jesus’ day, the most significant once-for-all kind of washing was proselyte baptism. Gentiles were usually baptized when they converted to Judaism; this was widely known and is even mentioned by the Greek philosopher Epictetus. By reporting that John asks Jews to be baptized in an act of conversion, the Gospel writers suggest that John treats Jews as if they are pagans, which was unheard-of…
A proselyte is a Gentile convert to Judaism. Therefore, baptism was a one-time conversion rite initiating a person into the community of God’s people. In Jewish thinking, by their ethnic heritage they were the holy people of God and the Gentiles were therefore unclean simply due to not sharing their Jewish heritage. Therefore, to become a Jew, which was more than a change in one’s religious belief but also a change in one’s identity including nationality and ethnicity, they had to be washed of their Gentile uncleanliness. John was preaching an eschatological (i.e. end time) message that God was returning to his people, bringing to an end the former world order and ushering in a new age. They therefore had to ready themselves. They automatically thought being Jewish by descent was enough to qualify for the status of being a member of God’s covenant people. John however said it was not enough (Matthew 3:9.) They could not rely on their heritage because God required true obedience from the heart so they needed as radical a change as a Gentile becoming a Jew (Matthew 3:7-10.). In the particular context of John, baptism was an initiation rite into an eschatological community: the faithful, end time people of God who will be saved.
Jesus adopted baptism from John, who was his forerunner, and he too carried on that eschatological vision of a renewed, covenantally faithful people of God. The difference was it was to be fulfilled in and through his person. John immersing people at the Jordan, the same river they had to cross when the nation first entered the Promised Land, meant they were truly becoming the covenant people, on the covenant land, obeying the covenant God anew. It was a true transformation. After Jesus was immersed by John, God said to him, “This is my beloved son, with whom I am well pleased.” This was a not a heavenly declaration of Jesus’ divine status. Though Jesus is clearly divine in the New Testament the title “Son of God”, as I have pointed out many times before, is not a divine title. The New Testament took the term from the Old where it primarily meant an Israelite, a faithful member of God’s covenant people, as well as a reference to the Messiah. Jesus is portrayed in the gospels and elsewhere in the NT as the new, faithful Israel (Matthew 2:15; Galatians 3:16; Revelation 21:12-14.) So the heavenly voice was a public declaration that Jesus was the embodiment of faithful Israel. John’s baptismal ministry was to point to Jesus, following him and being incorporated into him as the the true Israel. This carried forward to the early church through Jesus’ command where baptism was a conversion rite initiating them into the Church, the new Israel, the people of God in Jesus the Messiah (Ephesians 2:11-13.)
Just like with Gentile proselytes, through baptism you gained a new identity in the Messiah, becoming a new creature altogether (2 Corinthians 5:17.) Jews in the second Temple period sometimes described this change with proselytes as being “born anew”. As a Jew of that period, Jesus also used that language to describe what it meant to follow him (John 3:3.) While water baptism was a ritual sign of this dramatic change, John pointed to Jesus who would “baptise in the Spirit.” Though it was his followers, never Jesus, who administered the water ritual, he was the one who caused the dramatic, fundamental, spiritual change. Jesus would immerse people, cleansing them with God’s own Holy Spirit. This has various resonances in the Old Testament, particularly with passages like Ezekiel 36:25-27, which is found in the context of the redemptive renewal of the people of God and the fulfilment of the ancient covenant promises. In New Testament theology, those Jewish promises are fulfilled in the Messiah Jesus (2 Corinthians 1:20.)
Baptism, like many other things in the New Testament, has a thoroughly Jewish meaning. Remember, Christianity began as a sect within second Temple Judaism. When we forget its quintessentially Jewish roots we lose sight of what it all means. However, baptism, as a symbol of radical transformation, acquired a new meaning in the early church even though it still had earlier Jewish precedents. From an excerpt in an earlier post based on the work of NT Wright I write,
Even though resurrection was a belief in a literal event it was used metaphorically without eschewing its concrete referent. In post-exilic writing it was a metaphor for the restoration of Israel. Early Christian resurrection beliefs modify this, moving away from a nationalistic, ethnic focus to a new set of metaphors such as baptism and new birth. Again these new metaphors have a literal concrete referent in bodily resurrection. However, there is a radical change in resurrection symbolism from second Temple Judaism to early Christianity. New and different metaphorical meanings emerge in the early Jesus movement, shedding the old usages.
Simply put, baptism became a metaphor for the resurrection of Jesus. So to be baptised symbolically meant participating in the death and resurrection of Jesus, thereby being incorporated into him and the community of his followers.
In summary ritual washing of various forms was common in the ancient world of the New Testament. John came up with a prophetic, ritual innovation in immersion that had an eschatological focus. It was adapted by Jesus and his followers and after his resurrection acquired a new symbolic meaning in the early church. We can be certain that in the New Testament the conversion rite happened by being dipped into a body of water and was a fully cognizant choice so only those mature enough to make the choice participated in it. This however does not mean the New Testament necessarily affirms everything about modern credobaptism, that is the practice of only baptizing confessing adults. Baptism, that is, being dipped into water was a mandatory initiation rite into the community of believers and not a particular denomination. It was a symbol of unity and not our current fragmentation. This physical act became a metaphor for participation in the Messiah as well as spiritual regeneration by virtue of having a new identity in our resurrected Lord.
But when the goodness and loving kindness of God our Saviour appeared, he saved us, not because of works done by us in righteousness, but according to his own mercy, by the washing of regeneration and renewal of the Holy Spirit, whom he poured out on us richly through Jesus Christ our Saviour, so that being justified by his grace we might become heirs according to the hope of eternal life. – Titus 3:4-7 ESV
 βάπτισμα, Kittel, Gerhard; Friedrich, Gerhard; Bromiley, Geoffrey, Theological Dictionary of the New Testament (Abridged), p.93.
 Craig Keener, The IVP Bible Background Commentary: New Testament 2nd Edition, p. 251, IVP, 2014.
 Ibid., p. 255.