While clearly religious views play a big role in the oppostion to the disengagement, I would like to lay out the problems with the way the decision was made, from a non-religious point of view.
Israel is a list based party system. You do not elect a Prime Minister, you vote for a party list. The Likud party, which Ariel Sharon leads, platform in the last election was NO to unilateral disengagement from Gaza. The Labor party, led by Mitzna, platform was unilateral disengagement from Gaza. The results of the election were clear, Labor was trounced (40-19 in terms of seats). In other words the will of the people as expressed in the last election was NO to disengagment.
Furthermore, Sharon put the question to a referendum in his party, he lost. Given that there are no direct elections, the honorable thing to do would have been to resign. The party rejected his vision, therefore he should have bowed to the wishes of the party, after all, it was the party that was elected not Sharon. He refused to do that and he did not honor the results of the referendum.
In the Israeli system the Prime Minister is not sovereign, the government is (unlike in the US where the President has clearly delineated powers). The government would have voted against disengagement, Sharon had to fire ministers to pass the plan in the government. While that may have been strictly legal, it was certainly against the spirit of the system.
To top it all off, a number of left wing journalist published a book whose thesis is that Sharon cooked up disengagemnt to save himself from being indicted. The former Chief of Staff of teh Army testified in the knesset that he found about the plan from the press. This raises very troubling questions.
All in all, what Sharon did was stricly legal, but was certainly not democratic in the broader meaning of the word. Anyone with a shred of dignity and honor would have called new elections and said, I am running on the platform of disenagement. If as he and the left claims that a majority supports disengagement then he would have been re-elected with a clear mandate. This would have been much much healthier for the country. The right wing would understand that they lost a democratic election and the level of protest would be much lower. No one would be able to claim that the decision was illegitimate. Now, the right wing has a feeling that the decision was stolen and is illegitimate.
Disengagement is not a regular everyday government decision. It is a decision that affects everyone who lives in Israel, affects our right to the land, affects everyones future. It is only fair to give the people the right to decide on such a critical issue either with a referendum or even better, with new elections.