[Quoting A.W. Pink] This is a very solemn question, and we must restrict ourselves to what the Scriptures say by way of reply.
“You shall not listen to the words of that prophet or that dreamer of dreams. For the Lord your God is testing you, to know whether you love the Lord your God with all your heart and with all your soul”. (Deuteronomy 13:3).
From those words it is clear that God allows teachers of error for the same reason as He does persecutors of His people: to test their love, to try their fidelity, to show that their loyalty to him is such that they will not give ear unto His enemies. Error has always been more popular than the Truth, for it lets down the bars and fosters fleshly indulgence, but for that very reason it is obnoxious to the godly.
The one who by grace can say “I have chosen the way of Truth” will be able to add “I have stuck unto Thy testimonies” (Psalm 119:30, 31), none being able to move him therefrom.
“For there must be factions among you in order that those who are genuine among you may be recognized.” (1 Corinthians 11:19).
Error serves as a flail, separating the chaff from the wheat. Let some plausible and popular preacher come forward with an old error decked out in new clothes and empty professors will at once flock to his standard; but not so with those who are established in the Faith.
Thus, by means of the false prophets, God makes it appear who are the ones who hold the Truth in sincerity: they are faithful to Him despite all temptations to turn away unto a “broader-minded” way. The genuine gold endures every test to which it is subjected. Thus too are the unregenerate “converts” revealed: the counterfeit gold will not withstand the fire. Those who are attracted by a novelty do not endure but are soon carried away by some newer innovation.
HT Old Truth.