Who believed that the Crusaders and the Mongols defeated Islamic armies because of Muslim sin, and that Muslims needed to internally purify themselves?

Answer: Taqi al Din ibn Taymiyya

Explanation:

Taqi al Din ibn Taymiyya was Muslim theologian who lived between late 13th century to early 14th century. He was reported to have against the invasion and killing of Muslims by the Mongols. He believed the Muslims were defeated because they were sinful and needed internal purity.