Jude 1:4 gives us an ominous warning,
For certain men have crept in unnoticed, who long ago were marked out for this condemnation, ungodly men, who turn the grace of our God into lewdness and deny the only Lord God and our Lord Jesus Christ.
Jude is giving us an idea of how the devil brings error into a church. The key phrase is men have crept in unnoticed. That phrase literally means to come alongside by stealth. It pictures someone creeping in and coming alongside, like coming in through a side door.
The folks Jude is talking about look like everyone else, they talk like everyone else, but they are not like everyone else.
That is the way the devil brings error and false doctrine into the Church. It looks like the truth, sounds good, seems like the truth, and it may even be partially true. But there is enough poison in it to kill you.
It's like the guy who wanted to break into a used car lot to steal a bunch of auto parts. The only problem was the two guard dogs. So for the next week he showed up every night with some pieces of meat.
At first the dogs would bark like crazy, but after the man left, they would eat the meat. By the end of the week, they didn't bark at all, they just wanted the meat. So, having become familiar with the dogs, knowing that they wouldn't "sound the alarm," he approached them one last time—with poisoned meat. The dogs ate, and he was able to get into the lot and steal all he wanted.
That is the way the devil does it a lot of times. He sends someone among the believers in order to distract and detour them from the truth. But it is calculated and happens by degrees. Do not let your "inner alarm system" go silent through familiarity.
Beware of those who would move you away, even subtly, from the clearly revealed truth of God's Word.