The simplicity of the Gospel is this:
We can be more sure of our salvation in Christ than that the sun will rise tomorrow.
Consider that. It is more likely that the world will end in the middle of the night tonight and the sun will not rise tomorrow than salvation in Christ will not happen. And that is because our salvation has already been assured by the death and resurrection of our Lord Jesus. We are saved by our faith in Christ’s work, not our own. And to get this salvation, all we must do is believe in Him. That faith is given to us through the hearing of the Word and the work of the Holy Spirit in our hearts. So, it is this and this only:
Come, listen, believe and live!