Theophylact of Ohrid

Or else the Lord warns His disciples to be as light, in their life and conversation; as if He said, As a candle is put so as to give light, so all will look to your life. Therefore be diligent to lead a good life; sit not in corners, but be ye a candle. For a candle gives light, not when placed under a bed, but on a candlestick; this light indeed must be placed on a candlestick, that is, on the eminence of a godly life, that it may be able to give light to others. Not under a bushel, that is, in things pertaining to the palate, nor under a bed, that is, in idleness. For no one who seeks after the delights of his palate and loves rest can be a light shining over all.