John Chrysostom
'One esteems one day above another, while another judges every day alike.' Here, I think, the Apostle also hints slightly at fasting. Naturally, some of those who fasted constantly condemned those who did not fast. Or perhaps among those who discriminated in food there were some who observed this discrimination on certain days and not on others. For this reason the Apostle said, 'Let each be fully convinced in his own mind.' By this he freed those who discriminated from fear, calling their action indifferent. From those who attacked them too much, he removed the occasion for reproach. He instructed them that they should not constantly trouble people with such great zeal because of their discrimination.
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Such zeal was unnecessary not because of the nature of the matter itself, but because of the timing. Those whom they reproached were still new in the faith. In the Epistle to the Colossians he forbids this very strictly, saying, 'Beware lest anyone deceive you through philosophy and vain deceit, according to human tradition, according to the elements of the world, and not according to Christ' (Col 2:8). And again: 'Let no one therefore judge you in food or in drink,' and 'Let no one deceive you' (Col 2:16-18). In the Epistle to the Galatians he demands from them, with full strictness, a higher understanding and perfection in this matter. But here he does not resort to such insistence, since the faith had only recently been planted among the Romans. Therefore we too must not extend the Apostle's words, 'Let each be fully convinced in his own mind,' to all cases. When doctrine is in question, hear how he speaks: 'If anyone preaches to you a gospel contrary to what you received, even if he were an angel, let him be anathema' (Gal 1:8-9). Elsewhere he says, 'I fear lest, as the serpent deceived Eve, so your minds should be corrupted' (2 Cor 11:3). And in the Epistle to the Philippians he writes, 'Beware of dogs, beware of evil workers, beware of the mutilation' (Phil 3:2). But it was still too early to correct this in the Romans. Therefore he said, 'Let each be fully convinced in his own mind.' Since the discussion also concerned fasting, he overthrows the pride of some and drives out the fear of others by adding what follows.