Is it something new when false teaching slips into a church or denomination?
No. When false teaching slips into a congregation or denomination, it is nothing new. The writers of Scripture were already aware of it. Jude uses the same language of false brethren brought in unawares, showing this problem existed in the early church and was anticipated.
Les explains that in the passage under discussion, the next two words are "brought in." These false brethren did not sneak in; the leaders no doubt had something to do with it because they were brought in. Les suggests Peter was aware of these people, since someone influential brought them into the Jerusalem council meeting of Jewish believers to confront Paul and Barnabas, and Titus could be included, though it was predominantly against Paul and Barnabas. The same word is used in Jude, the one-chapter letter just before Revelation, down to verses 3-4. So when false teaching slips into a congregation or denomination, it is nothing new; the writers of Scripture were already aware of it.
Beloved, when I gave all diligence to write unto you of the common salvation, it was needful for me to write unto you, and exhort you that ye should earnestly contend for the faith which was once delivered unto the saints.
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