Why couldn't none of the human ideas be right?
Cause he does not believe in them and they where not given to us by half/full god born from a virgin impregnated by a magic sperm.
How can we be sure that God said to do it based on scripture?
Because he believes it and belief trumps reason every time.
Why would only some believe you to be right?
Because all the others hate god like Buddhist, or Gandhi, or your tipycal atheist
Christianity, One woman's lie about an affair that got seriously out of hand