Whoever believes and is baptized will be saved, but whoever does not believe will be condemned.
There is a simple blessing of faith; salvation. Many people have attempted to change the message of salvation into other things. Some have justified themselves by believing that they are a relatively good person and therefore should go to heaven.
Other people have convinced themselves that they’ve done enough good things or enacted enough penance to balance out their sin. Still other people refuse to believe in a God who would punish sin at all, instead choosing to ignore all of the warnings of punishment and holding to the idea that a loving God would not or could not send anyone to hell.
Mark 16:16 is simple and explicit as to what is required for salvation. In the end it is very little. We are called to have faith, or believe as Mark puts it. What do we believe? That Jesus died on the cross for our sins and that He rose again and that salvation is found only through Him.
Some people try to add baptism as a necessary requirement for salvation but they miss the point. If baptism were required, our salvation would be based on works which is exactly the opposite of faith. Baptism is a proof of our faith not a saving act. Our faith should show in all that we do but we are not required to prove our faith in order to actually be saved.