Are Buddhists Prohibited from Reading the Literature of Other Religions?
Are Buddhists Prohibited from Reading the Literature of Other Religions?In principle, not only are Buddhists not prohibited from reading the literature of other religions, but they are even encouraged to do so. Since Buddhists believe that faith in Buddhism accords with reason, it is impossible to convert any Buddhist who really understands the Dharma to another religion. For such a Buddhist, while propaganda from other religions may test his faith, it will not succeed in converting him. Moreover, Buddhists are not imperious towards other religions and do not deny their value. Buddhism divides the methods of transforming the world into five approaches or vehicles, and the teachings of the human and heavenly vehicle are foundational to all five vehicles and common to all other beneficial religions and philosophies in the world.  Therefore, Buddhism affirms the value of the scriptures of other religions, except for the arbitrarily judgmental, superstitious, and unreasonable parts.
While affirming the value of other religions, an orthodox Buddhist ought to be a propagator of the Buddhadharma. To enhance his skills in propagating the Dharma, to convert believers of other faiths to Buddhism, and to help people undecided between Buddhism and other religions, it is very important to gain some knowledge of comparative religion. If a Buddhist cannot demonstrate the superiority of Buddhism over other religions, how can he persuade others to submit gladly to faith in Buddhism? So an ideal Buddhist should have certain degree of knowledge concerning other religions. 
Of course, there is no need for a beginning Buddhist to study the doctrines of other religions. As a general rule, if a Buddhist has spare time, she is advised to spend no more than one-third of her time studying the writings of other religions. Otherwise, if she lacks the time to read about Buddhism, how could she have the spare time to peruse the literature of other religions?
- From "Orthodox Chinese Buddhism" by Chan Master Sheng Yen (p.153-154)
 See entry 4.15 [of book] for more on these five vehicles. Trans.
 In the "Monastic Code in Ten Recitations", scroll 38, the Buddha says: "From now on, in order to defeat outer-path ascetics, you may chant and read books of the outer paths (T 1435: 23:274b10). [Note that the Buddha made this statement after an outer-path practitioner won the upper hand in disputes with newly ordained Buddhist monks] In the "Monastic Code of the Mulasarvastivada, Miscellaneous Matters", scroll 6, the Buddha says: "Those who are ignorant, of little wisdom, or do not discern clearly should not study outer books. Only people who know themselves to be intelligent, learned, with sharp memories, and capable of defeating the outer-path advocates should study them… You should divide your time into three periods. Read Buddhist scriptures for two periods, and outer-path scriptures in the third period… In the first period of the day and in the period after noon, you can read Buddhists scriptures. When the evening period arrives, you should open the outer path scriptures" (T 1451: 24.232b5-13). Author