This answer is reposted from here.
I know someone who asked Rav Mordechai Willig about reading the Book of Mormon and he said no.
For further reading see the first mishna in perek cheilek (that would be the 10th perek in Sanhedrin) and in the Rambam and Bartenura on the line from R' Akiva, "Even one who reads seforim chitzonim [has no chelek to olam habah]". These meforshim explain that the problem is one of complete waste of time, and therefore the Bartenura says in maseches avos (5:22) that it is permitted to read these books in the restroom because there it is asur to think in Torah. According to this, given that everyone wastes so much time with useless stuff anyway, there would seem to be a strong leg to stand on from here to say it's permissible.
Also see shu"t Rashba chelek aleph simanim 414-418 (warning, 418 is very, very long) about what is seemingly the same topic where he is quite machmir.
There is also a teshuva of the Rivash (chelek aleph siman 45) which is very helpful, where he speaks out his view of chachmas yevanis (lit. Greek wisdom) as well as that of Rashi and the Rambam, ע"ש. He brings sources from chazal that chochmas yevanis is real bad, however, he describes it not as philosophical or theological works, but as speaking in riddles, ע"ש. Based on this as well it would seem to be permissible.
I also know that a prominent Rosh Yeshiva in E"Y read Thomas Aquinas in university for the purpose of writing a paper on the existence of God.
EDIT: However, see Yoreh De'ah siman 246 sif 4 and the Shach there sif katan 6 who bolsters the Rama and says not to study such things until one is well versed in all areas of Torah.
However, please do not rely on this, go and ask someone who actually knows what they're talking about.