Strenuous attempts to gain adherents...

Always test the spirits

4 Beloved, do not believe every spirit, but test the spirits to see whether they are from God, for many false prophets have gone out into the world. 2 By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God, 3 and every spirit that does not confess Jesus is not from God. This is the spirit of the antichrist, which you heard was coming and now is in the world already. 4 Little children, you are from God and have overcome them, for he who is in you is greater than he who is in the world. 5 They are from the world; therefore they speak from the world, and the world listens to them. 6 We are from God. Whoever knows God listens to us; whoever is not from God does not listen to us. By this we know the Spirit of truth and the spirit of error.  - 1 John 4: 1-6

Dr. Paul Kretzmann's commentary offers worthy insights on why we should always test what is taught us:

 
Good prophets are preachers through whom the Spirit of God, the Holy Ghost, teaches and preaches, whether it be by direct inspiration, as in the Old Testament, or whether it be by the teaching of the pure Gospel, as in the case of all true ministers today. In that sense they are spirits. But the Christians are here warned to use all care, to be on the lookout in ceaseless vigilance; for unfortunately not every man that claims to be a true prophet is able to present such credentials as the Word of God demands in such a case. These men, who presume upon the rights and duties of true Christian ministers, go out into the world, they display a remarkable missionary activity, they make the most strenuous attempts to gain adherents for their false tenets. Therefore Christians, as they value their soul’s salvation, must examine, test, such spirits and their doctrines, whether they are of God.
 

One of the fundamental facts of Christianity is the doctrine that Jesus Christ came into the world, became flesh. That is the touchstone which enables the believers to distinguish between true and false teachers. For in this doctrine is included the confession that Jesus Christ is the eternal Son of God, who, according to the promise given by God, became man, and by His vicarious suffering and death, and by His victorious resurrection and ascension merited our righteousness and salvation. He that accepts and confesses these truths unequivocally, with all that they imply, may be considered a preacher from God. But every professed teacher in the Church or outside of the Church that denies the incarnation of the eternal Son of God; that denies that Jesus Christ is our only righteousness and salvation; every one that teaches that we, in order to be saved, should not trust in Christ and in His merits alone, but also in our own works: such a man is not of God.
 

The apostle adds another reason for carefully examining the claims and for guarding against the evil influence of the false teachers: They are of the world, therefore they talk as of the world, and the world listens to them. No matter what their pretense and their glamour, the false teachers belong to the world, they have the world’s manner and mind. This is shown also in their talking, in their teaching and preaching, for its substance is not divine and leading to godliness, but it is inspired by the world, by its manner of thinking and acting. False teachers usually have messages that tickle the itching ears of their hearers. The children of the world will gladly hear them, the world receives their doctrines with enthusiasm. It is an almost unfailing criterion: if a certain preacher is widely advertised and acclaimed as a prophet for our times, he has probably managed to accommodate the old Scriptural language to some of his own philosophy in denying the fundamentals of the Bible. Witness the so-called Christianity of the social gospel.