"Go, therefore, make disciples of all nations; baptise them in the name of the Father and of the Son and of the Holy Spirit, and teach them to observe everthing I have commanded you." Matthew 28:19-20
Baptism is the first and most important of the seven Sacraments. Through Baptism, we are cleansed of our sin and are born again as a new person into the body of Christ.
If you were baptised outside the Catholic Church and wish to convert to Catholicism, you will not be baptised again. We only have one Baptism, as long it follows the valid formula laid out in scripture and tradition. Most mainstream Christian denominations celebrate valid Baptisms, but your parish priest will be able to confirm this for you. In this case, you will recieve the Sacrament of Confirmation, which is the second Sacrament. During Confirmation, your baptismal committment is strengthened with the gift of the Holy Spirit. You are annointed with Chrism oil and, if you have been baptised elsewhere, you enter in the Catholic Church.
If you have not been previously baptised, you will recieve the three Sacraments of Initiation during the same Mass. It is a special day, and will be the source of great joy for you and your loved ones.
For adults, Baptism marks a meaningful change in our lives. We are not only cleansed of original sin, but also the sins we have committed in our lives up to the point of Baptism. Water is poured over our heads three times, or you may be baptised by immersion, as the priest says, “I Baptise you in the name of the Father, the Son and the Holy Spirit.”
"Amen I say to you, no one can enter the Kingdom of God without being born of Water and Spirit." John 3:3
When you enter the water, it is a type of death. The old you dies, just as Jesus died on the Cross in order for your sins to be forgiven. When you emerge from the water, you are reborn, just as Christ emerged from the tomb having beaten death and sin for us all. In this way we are joined with Jesus in his Easter journey, from his death to his resurrection, and the indelible mark of Christ is with us forever.
"It was in those days that Jesus came from Nazareth in Galilee and was baptised in the Jordan by John. And at once, as he was coming up out of the water, he saw the heavens torn apart and the Spirit, like a dove, descended on him. And a voice came from Heaven, 'you are my Son, the Beloved; in you I am well pleased." Mark 1:9-11