Allaah (The Most High) said:

وَلَوۡلَا فَضۡلُ ٱللَّهِ عَلَيۡكُمۡ وَرَحۡمَتُهُ ۥ مَا زَكَىٰ مِنكُم مِّنۡ أَحَدٍ أَبَدً۬ا وَلَـٰكِنَّ ٱللَّهَ يُزَكِّى مَن يَشَآءُ‌ۗ وَٱللَّهُ سَمِيعٌ عَلِيمٌ۬

”And had it not been for the Grace of Allah and His Mercy on you, not one of you would ever have been pure from sins. But Allah purifies (guides to Islam) whom He wills, and Allah is All-Hearer, All-Knower.” [24:21]