Jamieson, Fausset and Brown on Hebrews 13:7

7. Two manifestations of "brotherly love," hospitality and care for those in bonds.

Be not forgetful--implying it was a duty which they all recognized, but which they might forget to act on (He 13:3, 7, 16). The enemies of Christianity themselves have noticed the practice of this virtue among Christians [Julian, Epistles, 49].

entertained angels unawares--Abraham and Lot did so (Ge 18:2; 19:1). To obviate the natural distrust felt of strangers, Paul says, an unknown guest may be better than he looks: he may be unexpectedly found to be as much a messenger of God for good, as the angels (whose name means messenger) are; nay more, if a Christian, he represents Christ Himself. There is a play on the same Greek word, Be not forgetful and unaware; let not the duty of hospitality to strangers escape you; for, by entertaining strangers, it has escaped the entertainers that they were entertaining angels. Not unconscious and forgetful of the duty, they have unconsciously brought on themselves the blessing.