Listen to this morning's class, "Entering a Church," here.
Follow along with the sources here.
For further research:
1) Read R. Shlomo Brody's brief summary of this topic in his A Guide to the Complex:Contemporary Halakhic Debates, here.
2) Read R. J. David Bleich's analysis here.
3) View the source sheet on this topic from the Olamot website here.