2 John 10-12 is made in response to false teachers. It should be added that by welcoming false teachers, especially those who (as John indicates) have perverted the true message, one does more harm than good, so that it would be adequate to say that the only real "opportunity" to do good is to rebuff the false teachers, lest they harm others.

Note that Paul says, "As we have opportunity...", the word being kairos referring to an occasion, or a set and proper time. This situation would not offer an "opportunity" to do good unto all men precisely because it would do more harm than good.

Beyond that, within the social context of 2 John, the reference is not to giving someone who is homeless a place to stay, but to giving an itinerant teacher a central headquarters from which to distribute his teaching. It is not as though they need someplace to stay and that one can do "good" by being charitable to them.

Problem: According to Jesus, we are supposed to love our enemies, bless those who curse us, and do good to those who hate us (Matt. 5:44). However, according to John, we are not to receive into our house or even greet anyone who comes to us and does not believe that Christ is come in the flesh. Which are we supposed to do?

Solution: We are supposed to follow both instructions. The apparent discrepancy between these directives arises from the fact that they are talking about two totally different situations.

In the passage in Matthew, Jesus is contrasting His own teaching to that of the Pharisees. The divine principle of love should be the guiding principle of one’s life. Even though some people are the enemies of God, He still allows the rain to fall upon their crops and causes the sun to shine on them. God treats the wicked with loving kindness. However, He never condones their wickedness. As Paul points out in Romans, the goodness of God is not a sign of His approval of their actions. Rather, the goodness of God is designed to lead to repentance (Rom. 2:4).

The passage in 2 John is not talking about someone who simply comes to visit. Rather, John is talking about false teachers who are deceivers (v.7) and who come to present their doctrines.

First, John is instructing the local church, and the individuals of the local church, not to extend hospitality to these persons, because that would imply that the church accepted or approved of their teaching. The people of the local church were directed not even to give a Christian greeting to them, lest this be misconstrued as an attitude of tolerance of their false doctrines. This was by no means a command not to love one’s enemy. In fact, following John’s directives would be the supreme act of love for one’s enemy. By clearly demonstrating an intolerance for false doctrine, it would be possible to communicate to false teachers that they needed to repent. On the contrary, if the church or individual were to extend hospitality to a false teacher, he would be encouraged in his position and take this action as an acceptance of his doctrine, or as a covering of his unrighteousness.

Second, it must be remembered that, in the early church, the evangelistic and pastoral ministry of the church was conducted primarily by individuals who traveled from location to location. These itinerant pastors depended on the hospitality of the people of a local congregation. John is directing the church not to extend this kind of hospitality to teachers of false doctrine. This is not contradictory to Jesus’ teaching. We are to love our enemies, but not encourage them in their evil deeds. We are to do good to them that hate us, but not to condone their wickedness. As Jesus said, we are to show ourselves to be children of our Father. In the very same Sermon on the Mount, Jesus went on to warn His disciples to beware of false prophets “whocome... in sheep’s clothing” (Matt. 7:15). John gave practical application to this warning, and thereby encouraged the local church to maintain its purity and devotion to Christ. Early Christian tradition supports this; St. Irenaeus records that the Apostle John himself fled from a bathhouse when the heretic Cerinthus entered, demonstrating this exact principle of not associating with those who distort the core truths of Christianity.