The term 'baptism' is a Greek word; it may be rendered into
Latin by 'mersia'. when we immerse any thing in water, that
it may be entirely covered with water. The word also carries
a spiritual aspect to it according to the bible as persons
undergoing such are said to be "born again".
In the Bible the act of baptism first occurred in relation the Israelites while in the wilderness, where it is said they were baptized unto Moses in the cloud and in the sea (1Co 10:2)
In the New Testament baptisms were performed by John the cousin of Jesus, who was referred to as John the Baptist, as he performed baptisms. These baptisms were performed in the river Jordan, with those being baptized first confessing their sins.
John the Baptist, was given the mandate to prepare the way of our Lord Jesus Christ, saying, repent for the kingdom of heaven is at hand. John the Baptist, baptized with water, but Jesus who was to fulfill the Law, after which those being baptized would be baptized not of water only, but of the Holy Ghost.
Jesus himself underwent the baptism, after which the Spirit of God came upon him. Many today wonder of the significance of the baptism. Some believe it is no longer relevant. Jesus said in John 3:3 - except a man be born again, he cannot see the kingdom of God. In that having been baptized one is no longer of the flesh, but is of the Spirit, and therefore should be operating in the Spirit.
Baptism, therefore, is not just a symbolic act, but is necessary in order to enter the Kingdom of God, and having been buried in the baptism of Jesus Christ in like manner should raise from death in sin to newness of life (Rom 6:3-14)