Nicodemus asked Jesus how a person can be born again when he has already grown old. Baptism by water is necessary if you want to be saved. You must be born again by the power of the Holy Spirit. Through this privilege, a free gift of God, he enters a new and Divine life - sharing in the life of God. He becomes a member of the spiritual Body of Christ, now having the mind of Christ in his thoughts, words and actions.
Baptism is a new birth.
When you baptised, your old nature and spirit die with Christ. They are buried with Christ. A new nature within your new spirit resurrects with Christ. At the moment of Baptism, God actually creates a new mind and a new spirit inside you. You are no longer the same, part of your creation having been changed.