Paul talks about false brethren in 2 Cor 11:26:
In journeyings often, in perils of waters, in perils of robbers, in perils by mine own countrymen, in perils by the heathen, in perils in the city, in perils in the wilderness, in perils in the sea, in perils among false brethren; — KJV, II Corinthians 11:26
Jesus says in the sermon on the mount:
(Matthew 7:12-20 [KJV])  Therefore all things whatsoever ye would that men should do to you, do ye even so to them: for this is the law and the prophets.  Enter ye in at the strait gate: for wide is the gate, and broad is the way, that leadeth to destruction, and many there be which go in thereat:  Because strait is the gate, and narrow is the way, which leadeth unto life, and few there be that find it.  Beware of false prophets, which come to you in sheep’s clothing, but inwardly they are ravening wolves.  Ye shall know them by their fruits. Do men gather grapes of thorns, or figs of thistles?  Even so every good tree bringeth forth good fruit; but a corrupt tree bringeth forth evil fruit.  A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit.  Every tree that bringeth not forth good fruit is hewn down, and cast into the fire.  Wherefore by their fruits ye shall know them.
False brethren do not adhere to the law and the prophets (verse 12). They take the broad way (verse 13). They have evil fruit (verse 12, 16-20) They are more numerous than the true brethren (verse 13).
How should we deal with false brethren