Learning other religions
Is there any issur involved in the study of other religions including Christianity from an academic perspective or if only the person is interested to learn about them.
Yes, this may be included in the prohibition of לא תתורו אחרי לבבכם, which Chazal tell us refers to learning heretical ideas. See Rambam Avodah Zara CHapter 2 and Sefer Hachinuch mitzva 387, who explain the guidelines of this prohibition. See Shulchan Aruch Yoreh Deah siman 334-42 that one who engages in such works may be excommunicated.
In general we are commanded to distance ourselves as much as possible from all Avodah Zarah.
The Rishonim even took issue with the Rambam engaging in the study of philosophy, see Teshuvas Rivash 45. See also Teshuvas Lev Avraham Siman 143.