Just as often, I receive comments denying that person could possibly be false. One of the most common rebuttals I receive regarding why someone can't be a false teacher is that "They preach Jesus". Or, "They use scripture."
First of all, it stands to reason that since satan masquerades as an angel of light, "Therefore it is not surprising if his servants also disguise themselves as servants of righteousness." (2 Corinthians 11:14-15).
We are supposed to test the spirits. (1 John 4:1). This means "Test, dokimazo, a term used in metallurgy to assess the purity of metal." (source)
In discernment, it is important to do three things when approaching the topic. First, actually read the bible. If one never reads it, one will never know that warning verse is in there. Second, stop and think about what it means. Lift your eyes from the book, slow down, and think about what the Spirit is telling us here when He inspired Paul to write this verse, and every word in it. "masquerades", "light", "ministers", etc. Third, put application on the ground. The bible is a book of intellectual depth, a historical document unparalleled, sensitive and beautiful poetry, but it is also a manual of life. We are to lift our eyes from it and look around and apply its words to what we do, say, and hear.
Now as for the actual application and detecting, and back to the 'oh but he can't be false, he preaches Jesus!' comments. Let's take a look at the beginning of the bible and the end. In Genesis 3 and Revelation 6 we are given examples of the two most successful deceivers the earth has ever and will ever see, satan, and the antichrist.
In Genesis 3 satan is introduced to us as subtle. In Genesis 3:1 the first thing said about satan is that he is subtle (crafty). In the bible, a thing mentioned first is the most important. So if there is a list of qualities, look at the first one, or if they are repeated, (ie, HOLY HOLY HOLY) it is the author's way of emphasizing. We have CTRL-B for bold font now, but in those days they simply mentioned the first thing first that the Author wanted emphasized. For example, though satan was the most beautiful of all the angels, (Ezekiel 28:12), this is not the first thing mentioned about him. The first mention is a negative quality, he is crafty.
In Daniel 8:25 we are also told that the coming antichrist will cause craft to prosper. In both cases the word is used in a negative sense. It means deceit, treachery and cunning.
We see a picture of all this in Revelation 6:2,
"And I looked, and behold, a white horse! And its rider had a bow, and a crown was given to him, and he came out conquering, and to conquer."
You see the antichrist rides a white horse. He is holding a bow (but no arrows). He wears a crown. So the antichrist is disguised. He is masquerading. He appears on a horse of a heavenly color (white), having kinglike qualities, (crown) and holding a bow but no arrows (an nonthreatening diplomat rather than a bloody warrior).
As to that last one, a bow but no arrow, a diplomat rather than bloody warrior, the devil and the antichrist use their voice to win the kingdom. The devil speaks, he doesn't kill directly. His words are his weapons.
The coming antichrist does it the same way. By speaking flatteries.
In his place shall arise a contemptible person to whom royal majesty has not been given. He shall come in without warning and obtain the kingdom by flatteries. (Daniel 11:21)
Why should it be any different for all of the mini-antichrists (1 John 2:22) to insinuate themselves, by flatteries, smooth speech?
For such persons do not serve our Lord Christ, but their own appetites, and by smooth talk and flattery they deceive the hearts of the naive. (Romans 16:18).
Of course they speak of Jesus (more often, they speak of God than Jesus, but anyway...) The second word out of satan's mouth was 'God.' False teachers will speak of God. They will speak of Jesus. They will use scripture. Testing the spirits does not mean you stop there. It means really listening. Then it means comparing what they say with the bible.
"These also are proverbs of Solomon which the men of Hezekiah king of Judah copied. It is the glory of God to conceal things, but the glory of kings is to search things out. As the heavens for height, and the earth for depth, so the heart of kings is unsearchable. Take away the dross from the silver, and the smith has material for a vessel; take away the wicked from the presence of the king, and his throne will be established in righteousness." (Proverbs 25:1-28)
Testing the Spirits
Sinclair Ferguson: What is Discernment?