A Weak Conscience

However not all men have this knowledge; but some, being accustomed to the idol until now, eat food as if it were sacrificed to an idol; and their conscience being weak is defiled. (1 Corinthians 8:4–7)

In verses 4–6 Paul states his agreement with the Corinthians who were theologically well taught. First he agrees that there is no such thing as an idol in the world. The stone, precious metal, or wood is real, but there is no god behind it. The image is not of anything that really exists. It only reflects the imagination of the one who designed it, or the impersonation of the demon who deceives through it (10:20).

Paul reminds them of an additional truth, one they must have known but that they did not take into consideration when exercising their Christian liberty. However not all men have this knowledge. Not all believers were mature in their knowledge and understanding of spiritual truths. Some were new Christians, freshly out of paganism and its many temptations and corruptions. They still imagined that idols, though evil, were real and that the gods the idols represented were real. They knew that there is only one right God but perhaps they had not yet fully grasped the truth that there is only one real God.

Even if they did understand that there was only one real God, the experiences of their past paganism were so fresh that they rejected all that was related to it. To participate in any way was to be tempted to fall back into former practices...

http://www.macarthurcommentaries.com