The Lord Jesus has made it clear to me that God considers all foods fit to eat. But if you think some foods are unfit to eat, then for you they are not fit.
and Peter said to them, "You know that we Jews are not allowed to have anything to do with other people. But God has shown me that he doesn't think anyone is unclean or unfit.
But if you do have doubts about what you eat, you are going against your beliefs. And you know that is wrong, because anything you do against your beliefs is sin.
You know all this, and so it doesn't bother you to eat in the temple of an idol. But suppose a person with a weak conscience sees you and decides to eat food that has been offered to idols.
Not everyone knows these things. In fact, many people have grown up with the belief that idols have life in them. So when they eat meat offered to idols, they are bothered by a weak conscience.
Everything is pure for someone whose heart is pure. But nothing is pure for an unbeliever with a dirty mind. That person's mind and conscience are destroyed.