This Sacrament can be received only once, because only once you can be born again in Christ by the power of the Holy Spirit, for the glory of the Father, for our own good, and for the good of the Church... and nobody can be baptized by himself, a member of the Churh has to baptize you!.
Thus, by Baptism, you become a Christian, and it gives me vertigo just to think what a Christian is: It is not just to imitate Jesus, which is very good, it is even more than a heart transplant, it is a new creature in Christ, not just a new heart, but a totally new creature!. The body and spirit of the person remains the same, with the same nose and hands and memory and intellect and will, and the same emotions and conscience and bad habits… but his soul is new!… now, as a whole, he is a new creature, becomes holy, pure, immaculate, a "child of God", a brother of Jesus, with the same inheritance than Jesus Christ himself, and the Holy Trinity living in him… he is now a tabernacle of Jesus, a dwelling of God the Father and the Holy Spirit, a Temple of God himself!.. and all of these, here, on planet hearth, right now! (Jn.14:23, 1Cor.3:16, 6:19, 2Cor.5:17). And he continues being the same "person": He is not a robot now, nor an animal with only instinct, he continues to be the same human person with free will. He still can love or hate, he can do his will or the will of God, he still can live with faith in Christ or live trusting in himself, the greatest sin of them all, worse than drugs or abortion or murder! (Rom.14:23). But if the Christian is faithful, in grace, with Christ in him, his hands and feet and heart and brain become those of Christ, so Jesus can teach the people, heal the sick, cast out the devil, feed the hungry of bread and love, give water to the thirsty, visit the sick and those in prison, cloth the naked... (Mk.16:17-18, Matt.25:31-46)... a most glorious adventure in Christ... and it is not I, but it is Christ who lives in me (Gal.2:20).
The baptized becomes a member of the Mystical Body of Christ, with the three distinctions of being a prophet, a priest, and a king, just like Jesus Christ!: A "prophet", to speak to men about God. A "priest", to talk to God about men. A "king", with power to cast out the devil and to heal the sick.
There are then two things involved in Baptism:
1- The immersion into water means the cleansing of all sins... we have died to sin, crucified and died with Christ, says Romans 6:1-8.
2- The coming out of the water means to be resurrected with Christ, to live with Him too, to live the life of Christ within ourselves (Romans 6:1-8), a most glorious adventure!.
Baptism is the fulfillment of the Scriptures: "I will sprinkle clean water upon you, and you shall be clean from all your uncleannesess, and from your idols I will cleanse you. A new heart I will give you, and a new spirit I will put within you; and I will remove from your body the heart if stone and give you a heart of flesh. I will put my Spirit within you, and make you follow my statutes and be careful to observe my ordinances" (Ezek.36:25-27).