What does 1 Corinthians 8:9 mean?

Paul has agreed on two points with those challenging his teaching about eating food offered to pagan idols. First, he agrees that food, in and of itself, is neither good or evil. It is just food (1 Corinthians 8:8). Paul has also agreed that the idols themselves are not actually gods. The entities they represent do not truly exist (1 Corinthians 8:4). He has noted that those who feel a pang of conscience over eating idol food do so thanks to a "weak" conscience (1 Corinthians 8:7).

Having said that, Paul will go on to show that God cares very much about why we eat and whether we eat with a clear conscience. To eat idol food in violation of one's conscience is a sin—not because of the physical meat, but because such a person is not convinced that God approves of that activity (Romans 14:23). Any action taken without conviction that it's allowed by God is, itself, a sin, even if the act would not otherwise be wrong. This is a crucial aspect of Christian morality which must be carefully understood.

Paul now warns those clear in their conscience about eating idol food. Through carelessness in exercising their right to eat, they might cause others to stumble. He uses the term proskomma, translated as "stumbling block." The word refers to something a person might trip over when walking, such as a stone or stick. Just because eating particular foods is not a sin for the "strong" Christian, that does not mean they should do so without any thought for the less-developed understanding of their brothers and sisters.

In other words, a Christian who is assured that eating idol food is acceptable is correct; however, they need to consider that their eating might place an obstacle in spiritual path of those with less developed consciences. To violate one's conscience is a sin, even if that conscience is "weak."