Second, the simplest predictor of terrorism is not Muslim or anything else. It's a question of whether foreign troops are stationed in your country. If you want terrorism, put your troops into someone else's neighborhood. Think that's inconceivable? Imagine the reaction in Dallas to Saudi troops stationed there. Or Boston's reaction to troops from Zimbabwe stationed around Faneuil hall. Or Russian troops ogling our girls at a San Diego beach.
Third, we have religious freedom and more fundamentally, freedom to believe what we choose even if it has nothing to do with - or is even fiercely opposed to - religion. This freedom is foundational to the modern world and bless Spinoza and Johann de Witt and then the Brits who followed the Dutch lead and then our founding fathers for enshrining that right of the individual to hold private beliefs and thoughts that don't require anyone else's approval or acceptance. Nothing. Nothing. Nothing is more essential to the peace, happiness and prosperity of a people than this. It's foundational to every good thing that follows. Every good thing.
Fourth, if you voted for Trump in spite of his disdain for religious freedom and ready willingness to jettison the first amendment in order to mollify people afraid of "others," you need to read some history. There is no limit to the grief that comes from regulating thought or religion and there may be no amendment to our constitution that has done more to fuel progress and happiness than the first amendment.
Fifth, I certainly believe that Trump may be kept from signing any laws that go against the first amendment and I think that is the most probable outcome. Partly because of other people and partly because of how wildly impractical it is to actually enforce laws based on what is inside of people's heads. The fact that you thought it unlikely he could make good on his promise is no excuse for giving him the power to attempt that. Trump has repeatedly demonstrated his ability to do what no one thought was possible.
This, by the way, is the first amendment in its entirety.
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.