Sometimes preachers, elders and other concerned Christians spend too much time on those who do not appreciate the gospel. We are often hesitant to "give up" on someone whom we believe to be a good prospect for the gospel. However, when our Lord sent out the apostles on the limited commission He said, "And whoever will not receive you nor hear you, when you depart from there, shake off the dust under your feet as a testimony against them" (Mark 6:11).
In the Sermon on the Mount Jesus said, "Do not give what is holy to the dogs; nor cast your pearls before swine, lest they trample them under their feet, and turn and tear you in pieces" (Matt. 7:6).
"The Christian must not be censoriously judicial, but he should be discriminatingly judicious. He must know dogs and swine when he sees them, and must not treat them as priests and kings, the fit objects for the bestowal of holy food and goodly ornaments. Dogs and swine were unclean animals. The former were usually undomesticated and were often fierce. In the East they are still the self-appointed scavengers of the street. The latter were undomesticated among the Jews, and hence are spoken of as wild and liable to attack man. Meats connected with the sacrificial service of the altar were holy. Even unclean men were not permitted to eat of them, much less unclean brutes. What was left after the priests and clean persons had eaten was to be burned with fire (Lev. vi. 24-30; vii. 15-21). To give holy things to dogs was to profane them. We are here forbidden, then, to use any religious office, work or ordinance, in such a manner as to degrade or profane it. Saloons ought not to be opened with prayer, nor ought adulterous marriages to be performed by a man of God. To give pearls to swine is to press the claims of the gospel upon those who despise it until they persecute you for annoying them with it. When such men are known they are to be avoided. Jesus acted on this principle in refusing to answer the Pharisees, and the apostles did the same thing in turning to the Gentiles when their Jewish hearers would begin to contradict and blaspheme." (J. W. McGarvey, The Fourfold Gospel, pp. 263, 264).
How can we tell when it is time to "give up" on someone and turn to other fields? It should be after we have taught, prayed and exercised all longsuffering -- but remember that even the longsuffering of God has limits (cf. 1 Pet. 3:20).
We need to realize that, despite our best efforts, many people will perish "because they did not receive the love of the truth, that they might be saved" (2 Thes. 2:10). Some people simply prefer their own way to the Lord's (Matt. 15:8-15). Other people will simply close their eyes to the truth (Matt. 13:15). Some Christians will fall away and it will be impossible for us to "renew them again to repentance, since they crucify again for themselves the Son of God, and put Him to an open shame" (Heb. 6:6).
The kingdom of God is precious indeed -- our Lord compared it to a treasure hidden in a field and a pearl of great price (Matt. 13:44-46). The Jews in Antioch of Pisidia rejected the gospel and judged themselves "unworthy of everlasting life" -- so Paul turned to the Gentiles (Acts 13:47). When people ask for us to leave them alone we need to respect their wishes and move on. I believe that we degrade the gospel when we take the very best that we have and lay it at the feet of swine!