4 Therefore concerning the eating of things offered to idols, we know that an idol is nothing in the world, and that there is no other God but one.
5 For even if there are so-called gods, whether in heaven or on earth (as there are many gods and many lords),
6 yet for us there is one God, the Father, of whom are all things, and we for Him; and one Lord Jesus Christ, through whom are all things, and through whom we live.
7 However, there is not in everyone that knowledge; for some, with consciousness of the idol, until now eat it as a thing offered to an idol; and their conscience, being weak, is defiled.
The point of verse 4 is that since there is only one God, things sacrificed to other non-existent gods mean nothing.
However (verse 5), although there are no other gods, there are those who worship what the think are gods represented by the idols (demons — 1 Corinthians 10:20). The idol is nothing, but the worship is evil because it is of demons and not of God.
Those who don’t fully understand about idols (v.7) or who, perhaps, used to worship idols, can be defiled by eating meat sacrificed to them — not because the meat is tainted, but because they eat against their own consciences.
defiled (v.7) = covered with mud