My kehillah has recently purchased a building for its shul. Until now, davening took place in rented accommodation, the lease for which is due to expire, with no prospect for renewal. The building my kehillah has purchased was previously used as a Christian Scientist house of worship. There were no tzelamim inside the building (in accordance with the tenets of this sect), and in general Christian Scientists maintain a very abstract conception of divinity. Some have claimed that the kehillah’s purchase of this building was improper, basing themselves largely on the source cited in Piskei Teshuvos 154:30. Unfortunately, the total destruction of the existing structure is beyond the means of the kehillah. Given that doctrines of the sect this building belonged to are far from those discussed by the classic poskim (as illustrated by the fact that their house of worship contained no tzelamim) together with the threat of “homelessness” the kehillah was faced with, is there room to say that they took the correct course of action in their purchase?
The answer to this question will depend on the alternatives available.
Assuming that this building was the best real estate option available, and that other similar opportunities were hard to come by, there is certainly room to say that this was the correct course of action (see sources, below).
Many authorities write that one should not convert a “house of idolatry” to a shul. This is recorded in the Eliya Rabba (154:15), Shut Bnei Zion (63), Pri Megadim (Eshel Avraham 17), Shut Chasam Sofer (Orach Chaim 42), Maharam Schik (Yoreh De’ah 154) and others.
Some refer specifically to the matter of converting a church (see Shut Bnei Zion, above), and discuss the matter of idolatry be-shituf — and are nonetheless stringent on the issue.
However, in the USA (in particular) it was common practice to convery churches into shuls, and this relied on the lenient rulings of the Magen Avraham (145:17) and Mishnah Berurah (45). Although Rav Moshe Feinstein (Orach Chaim 1:69) writes that this is not proper on a lechatchilah level, he agrees that bedieved it is fine.
Since the days of former Poskim, the nature of some Christian churches has changed dramatically. As the question notes, the nature of the church involved is such that there is no actual idolatry, and it seems that even the traditional concept of the Christian trinity is far from the “shituf” that the strictly Catholic tradition espouses.
Therefore, if there is room for leniency (under extenuating circumstances) for converting a regular church into a shul, there is all the more so room for leniency, even on a lechatchilah level, for converting the church noted in the question into a shul.
Although if there is a readily available alternative, there remains room to prefer it; however, in the absence of a similarly attractive alternative, it appears that the decision was correct and legitimate al-pi-halachah.