Adam Clarke Bible Commentary

One believeth that he may eat all things - He believes that whatsoever is wholesome and nourishing, whether herbs or flesh - whether enjoined or forbidden by the Mosaic law - may be safely and conscientiously used by every Christian. Another, who is weak, eateth herbs - Certain Jews, lately converted to the Christian faith, and having as yet little knowledge of its doctrines, believe the Mosaic law relative to clean and unclean meats to be still in force; and therefore, when they are in a Gentile country, for fear of being defiled, avoid flesh entirely and live on vegetables. And a Jew when in a heathen country acts thus, because he cannot tell whether the flesh which is sold in the market may be of a clean or unclean beast; whether it may not have been offered to an idol; or whether the blood may have been taken properly from it.