The passage you cite is Hebrews 13:2: “Do not forget to entertain strangers, for by so doing some have unwittingly entertained angels.” The passage does not tell us enough to know what incident was in the author’s mind, but there are several possibilities. Most likely is the time when Lot entertained the strangers who came to Sodom (Gen. 19:1-3). According to the account, Lot was sitting at the gate of the city one evening when two unidentified strangers came along. Without knowing who they were, he invited them to his home, for a night’s rest, and later learned that they were angels.
The author of Hebrews is emphasizing the need for hospitality; however, the practice of entertaining total strangers without any knowledge of their background has never been an automatic obligation. The mention of hospitality assumed that the recipients were Christian brethren, though they might not be one’s acquaintances. We today would not want to invite into our homes just any two strangers who might happen to come by.
Hospitality was commanded in other passages of Scripture. It was a plan set up because in the days of the early Church there were no readily available accommodations, and travelers had to rely on the helpfulness of friends. It was a service to which some people would naturally incline much more than others. Some who are of an outgoing, generous, affable personality might be very glad to help Christian travelers; others might be naturally suspicious. The apostle Peter advised–even commanded–the believers to “Be hospitable to one another without grumbling” (1 Pet. 4:9). And Paul advised Titus that a good bishop would be “hospitable, a love of what is good” (Titus 1:8), and he wrote to the Romans that they should be “given to hospitality” (Rom. 12:13).
Paul himself depended heavily on the hospitality of brethren as he traveled from place to place. Jesus also taught the principle of being willing givers: “Freely you have received, freely give” (Matt. 10:8). Again, “He who sows sparingly will also reap sparingly, and he who sows bountifully will also reap bountifully. So let each one give as he purposes in his heart, not grudgingly or of necessity; for God loves a cheerful giver. And God is able to make all grace abound toward you, that you, always having all sufficiency in all things, may have an abundance for every good work” (2 Cor. 9:6-8).
No one lives to himself, and none of us can provide entirely for our own needs. We are all to a greater or lesser extent dependent upon each other. At the same time, persons sharing a common interest (within the body of believers) will be glad to share common resources. The world will “love its own” (John 15:19), and so will the body of believers.
When God was working openly, it was possible to see angels and not know it. Today such would be impossible. No unidentified stranger today could be an angel, because angels are not visible to us now. However, we might, by entertaining a Christian brother or sister, be entertaining one who will one day become an angel.