'But take heed lest by any means this liberty of yours become a stumblingblock to the weak. For if a man sees you who have knowledge sitting at meat in an idol's temple, will not his conscience, if he is weak, be emboldened to eat things sacrificed to idols? For through your knowledge he who is weak perishes, the brother for whose sake Christ died. And thus, sinning against the brothers, and wounding their conscience when it is weak, you sin against Christ.'
We may ourselves be 'at liberty', be free from all superstition, free from all recognition of idols, but we should not thereby use our knowledge in such a way as to be a stumblingblock to the weak. We should ask ourselves, how will this affect others? In all things love must override everything else. For if we participate of idol meat in the Temple the weaker brother might see us, and knowing our spiritual position, and what he sees as our spiritual superiority, may himself feel that he can participate, his conscience satisfied because we have eaten, but it then result in his harm. For he may then consider himself as again involved in idols and be dragged down and defiled. He not having the strength to remain uninvolved.
So through our 'knowledge' the weaker brother, for whom Christ died, may perish (compare here Romans 14:15 which speaks of ‘destroying him for whom Christ died'). Thus we, by sinning against our brother and weakening his conscience, will actually be sinning against Christ.
'May perish.' The thought here is that this is 'a brother for whom Christ died'. Note that it is not 'a brother who is in Christ'. As with the community of Israel of old where there were included in 'the people of God' those outwardly dedicated to the covenant, whether inwardly so or not, so that the community was composed of both the true people of God and those who were only so outwardly, so in the New Testament too the church from one aspect was seen as including all those who outwardly believed and had been baptised, and included those whose true faith made them in Christ, and those who were bordering on being so, and could be seen outwardly as 'brothers', but could slip back and perish because they were not yet fully 'saved'. They had responded to the Christian message, they were learning and entering into faith, but they had not yet received full faith. They had 'believed in Christ' rather than 'into Christ' (compare John 2:23-25 and often). Christ died for all, but not all finally came.
Others would, however, argue that the sin is against Christ (Acts 9:5; Acts 22:8; Acts 26:15) precisely because the brother is in Christ. They see the idea as rather being that he will slip back and perish physically (compare 1 Corinthians 11:30) or possibly be spiritually shipwrecked and left adrift. He will be ‘destroyed' (Romans 14:15).