Moulana Ashraf Ali Thanwi رحمه الله once mentioned,
“If an irreligious person says something about Deen, zulmat (darkness) will be mixed in his words. Some zulmat will also be attached to the script of his writings. On the other hand if a Deendaar (righteous) person even speaks something (permissible) about dunya, there will be noor in his words. The reason for this is that words emanate from the heart. Hence the condition of the heart will definitely have an effect on one’s speech. Since the condition of the heart of the speaker and writer definitely has an effect on his speech and writings, therefore one must never study the books of irreligious people. Reading a book is like sitting in the company of the author. The evil effects that come upon one from sitting in the company of irreligious people will come from studying their books as well.”
(Madrasah Taleemudeen, Abnaa-e-Qadeem Newsletter, issue #2)