Entering a Church

 Shulchan Aruch YD (150:1) Entering an actual house of avodah zarah is clearly prohibited.

Igros Moshe (Y.D. 3:129.6) Question: Is it permitted to enter a church? Answer: It is clearly prohibited to enter a church which is a place of worship even if only to look at things made solely for decorations which  Tosfos says there is no prohibition to look at them as there is to a place of idolatry or the idols themselves. However a building where they actually worship is prohibited under all circumstances  The status of a courtyard where no worship takes place which serves merely as a transit before a house of worship and the concern is arousing suspicion of others is permitted. Also this that Tosfos says that the things made solely for decoration are permitted, this means decorations made for non religious purposes such as statues of the kings or forms on coins or pictures or illustration in manuscripts. However all that is made to enhance idolatry or the place of idolatry is prohibited to benefit or obtain pleasure from as idolatry itself. Aside from the prohibition of idolatry there is harm caused to faith by the mere desire to go there. In addition my son Rav Dovid said that the decorations were made solely to seduce and attract people to idolatry and that is clearly prohibited to let them accomplish their desire. Therefore G-d forbid! That anyone should go there.