If you whittle Christianity down to its very crux, it is this:
God sent his only Son, Jesus Christ, to die on the cross for our sins. Those who believe and trust that Jesus died, defeated death, and rose from the dead, are saved.
Islam denies God had a son. In fact, the Koran specifically states Jesus did NOT die on the cross.
These two beliefs cannot be reconciled. To deny that Jesus died on the cross, and then rose from the dead, is to deny the very core of Christianity.