I've heard Muslims say Christians altered the book of Genesis and switched the names of Isaac and Ishmael so that God's promises to Ishmael would show up as promises to Isaac. Perhaps these people didn't know what they were talking about...but I've heard this from different Muslims at different times.
Anyway, in the Dead Sea Scrolls, which contain many manuscripts of Genesis before Jesus' time (before Christians existed), why don't you find this name switch? How could Christians go back and change all of those?
And why don't Jews accuse Christians of switching the names?
It seems to me that the names were not switched...God's covenantal promises were indeed through Isaac, not Ishmael.