Hi! Here's more from the Catholic side of things! You cannot be un-christened, but neither does baptism guarantee entry into heaven. Baptism only makes entry into heaven possible. It is our response (of course, initiated and aided by God's grace) that brings us to salvation.
As Christians, none of us can know, for sure, whether or not we are saved. We all need to persevere until the end! St. Paul certainly did not believe he was saved and always saved! He wrote:
(1 Corinthians 9:26-27) "Therefore I do not run like someone running aimlessly; I do not fight like a boxer beating the air. No, I strike a blow to my bodyband make it my slave so that after I have preached to others, I myself will not be disqualified for the prize."
I could give you many more Scripture passages, such as:
(Hebrews 10:36) "You need to persevere so that when you have done the will of God, you will receive what he has promised."
(James 1:12) "Blessed is the one who perseveres under trial because, having stood the test, that person will receive the crown of life that the Lord has promised to those who love him."
(2 John 1:8-9) "Watch out that you do not lose what we have worked for, but that you may be rewarded fully. Anyone who runs ahead and does not continue in the teaching of Christ does not have God; whoever continues in the teaching has both the Father and the Son."