For if any man (i.e., any of the weak brethren) see thee which hast knowledge. — The fact of your being avowedly advanced in the knowledge of the faith will make your example the more dangerous, because more effective.
Sit at meat in the idol’s temple. — Some went so far as to not only eat, but eat in the precincts of the heathen temple. The Apostle being concerned now only with the point of the eating, does not rebuke this practice here, but he does so fully in 1 Corinthians 10:14-22. He probably mentions the fact here as an instance in which there could be no salving of his conscience by the heathen convert thinking that it was not certain whence the meat had come.
Be emboldened. — Better, be built up. The people addressed had probably argued that the force of their example would build up others. Yes, says St. Paul, with irony, it will build him up — to do what, being weak, he cannot do without sin.