In the Scriptural verses I've been writing about -- "If a peaceful person lives there, your peace will rest on him; but if not, it will return to you"; "love your enemies, and pray for those who persecute you, that you may be children of your heavenly Father"; "if your enemy is hungry, feed him; if he is thirsty, give him something to drink; for by so doing you will heap burning coals upon his head" -- I detect some hints about Christian communion.
In the first quotation, Jesus tells His disciples they needn't worry about their peace resting on those who aren't fit for it. In the second, He tells them to be like God in showering love upon everyone, regardless of their fitness. The proverb can (I think) be read as teaching that loving your enemy is a means of inviting him into the New Covenant.
In each case, there is at least an implicit concern that a Christian might be too free with the gifts Christ has given him to distribute. In each case, there is reassurance that he needn't worry, that he is to give freely and leave the bookkeeping to God.
In the Sermon on the Mount, Jesus prefaces His call to love our enemies with the words, "You have heard that it was said, 'You shall love your neighbor and hate your enemy.'" Under the Mosaic Law, the distinction between who was under the Law and who wasn't was clear. A Jew, at least at the time of Jesus, was required to treat Gentiles in a markedly different way than other Jews. Whether someone was a neighbor, whether someone was Jewish, was critical knowledge. For a Jew to cast pearls before swine was not only to waste the pearls, but to become unclean himself.
In the fulfillment of the Law, though, such concerns are all but eliminated. No act of love can be misdirected, and if the distinction between believer and non-believer is still important, it is not the cause of anxious separation it was under Moses. It seems to me the concern is more to avoid the company of sinners than of non-believers -- and, for that matter, an evangelizing faith can hardly counsel avoiding non-believers generally.