A christian cannot say that the source or moral truth is God, or that there are no absolute morals without God.
Let me explain.
If a christian were to say raping and torturing a child is morally wrong he is missing the one part that should be added to the end that allows for the child to be raped and tortured if God commands him or her to do so. The jews on their way out of exodus were commanded by God to kill and rape all the other tribes that stood in their way to the land of Israel, the land that God gave to them. It was morally right for the jews to do so at that time and therefore it's possible that God can command some people again to kill other people.
So, the christian must say that killing and raping a child is morally wrong except when God orders you to do so. So basically when God orders you to do so, then everything is permitted.