When Pharaoh demanded that Moshe Rabbeinu make the plague of hail stop, Moshe left the city in order to pray to Hashem to bring it to a halt. Rashi, quoting the Medrash, explains that he did not pray inside the city because it was full of idols. If so, why doesn’t the Torah tell us that he went out of the city when he prayed for the other plagues - the frogs and the wild beasts - to stop? The Netziv, R’ Naftali Tzvi Yehudah Berlin zt"l, answers that obviously, during the other plagues, Moshe prayed indoors, and the idols did not pose a problem. Only during the plague of hail did he pray outdoors, so that the miracle should be clear for all to see, and those hail stones that were still in the air when the plague came to a halt, did not fall to the ground. 

     A number of years ago, there was a shul that was situated next to a church, and members of both congregations were having difficulty finding ample parking. So they decided to make an agreement whereby synagogue members could leave their cars in the church parking lot over Shabbos, and church members could park their cars in the shul parking lot on Sunday. The Jews consulted R’ Dovid Feinstein zt"l as to whether this was permitted. He said that indeed they were allowed to let the gentiles park in their parking lot since this does not constitute, “helping someone commit a sin,” as they were going to church anyway. However, a Jew may not park in the church parking lot, because of a different problem. The Gemara teaches (עבודה זרה יב): “If a person gets a splinter while walking past an idol, he may not bend down and remove it, for it would appear that he is bowing to the idol. If it does not appear so, it is permitted.” 

     Concluded R’ Dovid, since a Jew would have to duck down while getting into his car, he would be giving the impression that he was bowing to the large "צלם" (crucifix) on the front of the church building, which has the halachic status of an idol. Thus, it is forbidden.