Or what fellowship is there between light and darkness? For we are the temple of the living God; as God said, "I will live in them and walk among them, and I will be their God, and they shall be my people.
17 Therefore come out from them, and be separate from them, says the Lord, and touch nothing unclean; then I will welcome you, 18 and I will be your father, and you shall be my sons and daughters, says the Lord Almighty." The background of this verse comes from an Old Testament instruction in Deuteronomy: You shall not plow with an ox and a donkey yoked together.
The reference to temples and idols suggests that Paul is still addressing the Corinthians' tendency to try to blend the worship of God with the activities that went on the pagan temples.
In other words, the people wanted to be Christian while still partaking of all the activities that marked the worship of the Greek gods.
The idea of separating and setting boundaries, even among plants and animals ("each after its kind"), expresses the idea that there is a certain way God's world should work, that there are certain boundaries and limits within which creation can exist (see God and Boundaries).