I was recently told that nowhere does Jesus promise assurance of salvation outside of our perseverance. One of my favorite verses on our assurance of salvation and the simplicity of the gospel message comes out of I John. “These things I have written to you who believe in the name of the Son of God, that you may know that you have eternal life. . . .” I John 5:13a. Adding our perseverance to the equation seems to be adding “works” to our salvation, or at least to maintaining our salvation.
I agree with you. Jesus made several statements confirming that our salvation rests solely upon our belief. John 3:16, John 6:28-29, John 6:40, are three examples. In addition Paul wrote that our salvation was guaranteed from the moment we believed, and no power in Heaven or on Earth can ever change that (Ephesians 1:13-14, Romans 8:38-39). And in 2 Cor. 1:21-22 he said in effect that God has assumed responsibility for us. He anointed us, set His seal of ownership on us, and put His Spirit in our hearts, guaranteeing what is to come.