Baptism is a pretty important part of being a Christian. Jesus commanded it (Matthew 28:19).
Rephrasing your initial question, is a person guaranteed to go to Heaven? Answer, no. Salvation, through Christ’s Passion and Resurrection, is offered as an unmerited, and un-earnable gift. The actions of Jesus can never be negated, and salvation is available for those is wish it. However, a person can refuse the gift of salvation, by living wickedly, disobediently, and unrepentant, and go to Hell.
Consider the unforgivable sin, blasphemy against the Holy Spirit. (Mark 3:28-29, Matthew 12:31-32, and Luke 12:10) This total rejection of salvation is a first class ticket to eternal damnation.
Is salvation then lost? No. It, and God’s accompanying mercy, are totally rejected. However, should the person realize the gravity of rejection, and truly seek God’s forgiveness, they are accepting the gift of salvation.
But once they accept, can they then reject it? That is the question being asked.