Surah 6. Al-An'am, Ayah 107

I.e., no mortal has it in his power to cause another person to believe unless God graces that person with His guidance.

God's Plan is to use the human will to co-operate in understanding Him and His relations to us. This is the answer to an objector who might say: "If He is All-powered, why does sin or evil exist in the world? Can He not destroy it?" He can, but His Plan is different, and in any case it is not for a Teacher to force any one to accept the truths which he is inspired to preach and proclaim.