It has nothing to do with the theology of Christianity. It has to do with the replacement of Islam which is necessary to end the threat of the barbarians. What would you replace Islam with? Humanism? Replacing religion with purely rational pap will not take with a population that has not grown up in the Northeastern US suburbs and gone to Harvard. It is not realistic to determine to send them all to Harvard. Forcible conversion is far superior to genocide which is the rational alternative for removing the threat from our children's future.
You wouldn't have to replace Islam with Christianity, if they were crushed and felt that Allah would not protect them.
They would abandon it themselves.
Forced conversion to Christanity is a antithetical proposition. You'll have to come up with a different plan. One that makes sense.