What is the meaning of the phrase "ولا نزكي على الله أحدا" (and we do not sanctify anyone before God)?

The meaning of the phrase is that judgment is based on outward appearance, while the true nature of the matter and its outcome are left to God, because the outward may or may not conform to the inward. Therefore, one should say: "I consider him to be so, and I do not purify anyone before God," and "And God Almighty knows best," to disassociate oneself from self-praise and claims of knowledge.