If Christians believe that Jesus sacrificed himself for their salvation, i.e. they are guaranteed heaven...?

If all it took was to believe that Jesus was sacrificed for our salvation, then that would qualify Satan to go back into Heaven. "You believe that there is one God. Good! Even the demons believe that - and shudder" (James 2:19). Belief is not the only requirement for Salvation. It takes admitting that you are a sinner, believing that God will save you, and confessing your sins to God. Also, there are other things that must be done. The list is in the link in the source section. All that was just to clarify a Christian's Salvation. Now to get to your question. Christians are not born Christian. When humans first sin, they displease God and have to ask Christ to save them to become a Christian. From then on, the Christian cannot rest. Salvation is not a thing of the past; it is an active process, but that does not mean that it can be taken away. As Christians, our job now is to help others receive Salvation, for it is not God's Will that any man should perish.