Anyone who questions Abraham's faith needs to have their head examined. For a Christian to do so is ironic given that Abraham's faith in God was greater than Jesus' was. Abraham never asked God why He has forsaken him.
"And about the ninth hour Jesus cried with a loud voice, saying, Eli, Eli, lama sabachthani? that is to say, My God, my God, why hast thou forsaken me?" Matthew 27:46 - King James Version
Abraham did not just bear a child with Hagar. He married her first. Therefore, his child with Hagar was legitimate. There is not another person mentioned in the Bible, including Jesus who can get anywhere near the relationship that Abraham has with God. Who else would migrate his wife and infant child to a desert land because he was directed by God to do so. Who else would be willing to kill his ONLY son Ishmael who he got at the advanced age of 86 when he was 99 years old and Ismael was just 13. (And yes, this event took place the year BEFORE Isaac was born). The person who did those things is Abraham, the true friend of God, the most impressive person mentioned in the Bible.