It is faith in the redemptive work of Christ that produces salvation - not our actions. James writes that faith without works is dead (James 2:20).

Faith alone saves, but saving faith is never alone; it must be acted upon.

This is what Mark is referring to when he speaks of baptism.

Water baptism is a command of Jesus and is the initial action taken upon believing. Mark's statement could be said this way, "He who believes with saving faith (i.e. faith that produces actions) will be saved." 

Water baptism is very important. It is an opportunity to act on your new profession of faith. Anyone who refuses to follow Jesus' command to receive water baptism, may be suspected of not really believing.

Water baptism is a command, but keeping this command does not produce justification. Jesus forgave sins without any mention of water baptism.

Philip told the Ethiopian eunuch, who asked him to baptize him, that if he believed with all of his heart, he could be baptized (Acts 8:37). Philip used water baptism only after an individual believed. This is also how Mark used water baptism.