Acts 10:1-29
Cornelius was a Gentile but he was also an uncircumcised worshiper of the one true God. He was a devout, generous, prayerful man who was respected by the Jewish people but one who, according to Jewish law, was unclean.
The Anchor Yale Bible Dictionary explains the concept of clean/unclean this way:
These terms designate negative or positive ritual and moral conditions in persons and sometimes in objects. Uncleanness or impurity is basically defined as that which is a threat to or opposes holiness, and hence must be kept separate from that sphere. On the relationship of purity and impurity to the profane (i.e., nonholy or common) and holy. Impurity may arise from natural and necessary conditions, such as bodily discharges, or from sinful situations.[1]
Basically, unclean means unworthy. If Jewish law said a thing or a condition was unclean, it meant that it was unworthy of being in God’s presence. It made a person unworthy of worshiping God and made anyone who came into contact with the thing or the person unworthy as well.
The Jews considered Cornelius unclean. Jewish law and traditions said it didn’t matter how devout, generous, or prayerful Cornelius was, he was unworthy of coming into God’s presence and unworthy of worshiping God. Because of this, Peter, had it not been for his vision, would have thought Cornelius unworthy. He might have thought well of him and respected him as a man but would not have thought of him as a child of God.
But God gave him the vision to change his thinking. In the vision, Jesus told him, “Do not call anything impure that God has made clean.”
[1] Freedman, David Noel, ed. “Unclean and Clean.” The Anchor Yale Bible Dictionary 1992: 729. Print.