[teejay]

>1. According to Christianity, was Ishmael the son of Abraham
>an illegitimate child?

Now how do you read coach? The bible which you previous in your own word have admitted perusing, says that God made one man for one woman. Do you think Abraham was right to marry another woman and father children with him?
Nevertheless he did it on grounds that His Wife Sarah was unable to bear children. This they did after consultations. Not what Sarah is telling him Abraham….
“And Sarai said unto Abram, Behold now, the LORD hath restrained me from bearing: I pray thee, go in unto my maid; it may be that I may obtain children by her. And Abram hearkened to the voice of Sarai.” Gen 16:2
Through this means therefore Ishmael was Abraham’s son, I would say though Illegitimate in the eyes of God.
Not it was not God but an agreement between wife and Husband.

>2. Which of the sons of Abraham (Ishmael or Isaac) was to be
>offered as sacrifice according to the Christian faith?

Of course Isaac….. “And he said, Take now thy son, thine only son Isaac, whom thou lovest, and get thee into the land of Moriah; and offer him there for a burnt offering upon one of the mountains which I will tell thee of….”