Paul outlined the basic facts of the gospel in 1 Corinthians 15 and told us there that it was possible to believe in vain. Like Jesus, Paul taught that you have to stay with the Word to be saved. You must hold fast to and receive the gospel. God made it simple, but Catholics, Orthodox, Calvinists and others make it extremely complicated to the point where you are led to believe that you can't understand it unless you have a Doctorate in Theology.
In this video I explain what the gospel is. For fuller explanations read Matthew, Mark, Luke and John.