In my current game, all lands have de jure drifted out of the Kingdom of Jerusalem--two crusades were called for Jerusalem, and one failed spectacularly while the other was abruptly ended inconclusively (the owner of the lands had an invasion happen to them), so they've been in Muslim position for a while. This seems unrealistic to me, because it makes Muslim ownership seems as legitimate as Christian ownership of the same lands, which most medieval Christians certainly wouldn't agree with.
I propose that there be separate de jure systems for Muslims and Christians. Jerusalem may drift in and out of this or that sultanate, which has effects on Muslim politics, while it should remain a part of the Kingdom of Jerusalem for Christians unless it de jure drifts into a different Christian kingdom. Prolonged Christian ownership of Jerusalem could be seen as legitimate, whereas prolonged Muslim ownership of Jerusalem would still be thought of simply as "Muslim occupation of the Holy Land".
On a side note...how can territory drift out of a kingdom that has never existed yet, anyway?
I propose that there be separate de jure systems for Muslims and Christians. Jerusalem may drift in and out of this or that sultanate, which has effects on Muslim politics, while it should remain a part of the Kingdom of Jerusalem for Christians unless it de jure drifts into a different Christian kingdom. Prolonged Christian ownership of Jerusalem could be seen as legitimate, whereas prolonged Muslim ownership of Jerusalem would still be thought of simply as "Muslim occupation of the Holy Land".
On a side note...how can territory drift out of a kingdom that has never existed yet, anyway?