No contradiction at all. If repenting and believing the gospel is the only way to be saved, but someone is incapable of doing so, how can they be held accountable? I leave that up to a fair, just God. In regards to mentally handicapped people, I've seen them at church praising God during worship, so how do you know where their heart is even if they come across with poor communication and appear to be dumb and clueless? The lamb of God does take away the sins of the world, but that does not mean that everyone will "automatically" be saved and inherit eternal life (John 3:18) so your doctrine is easy to dismiss.I didn't know that the scriptures only talk about salvation for adults who are capable of repenting. To me, it is very simple; the lamb of God who takes away the sins of the world. Your doctrine on salvation doesn't answer questions on salvation, so it is easy to dismiss it. On one hand you say repentance and believing the gospel is the only way a person can be saved, yet you still believe there are people (children and other special groups) who don't believe the gospel and don't repent but are still saved. this is a contradiction.
So if you were standing at the gates of heaven right now and Jesus Christ asked you why He should let you into heaven, what would be your exact answer?