Leviticus 10:1-3 AV
“1 And Nadab and Abihu, the sons of Aaron, took either of them his censer, and put fire therein, and put incense thereon, and offered strange fire before the LORD, which he commanded them not. 2 And there went out fire from the LORD, and devoured them, and they died before the LORD. 3 Then Moses said unto Aaron, This [is it] that the LORD spake, saying, I will be sanctified in them that come nigh me, and before all the people I will be glorified. And Aaron held his peace.” 

Numbers 3:1-4 AV
1  These also [are] the generations of Aaron and Moses in the day [that] the LORD spake with Moses in mount Sinai. 2 And these [are] the names of the sons of Aaron; Nadab the firstborn, and Abihu, Eleazar, and Ithamar. 3 These [are] the names of the sons of Aaron, the priests which were anointed, whom he consecrated to minister in the priest’s office. 4 And Nadab and Abihu died before the LORD, when they offered strange fire before the LORD, in the wilderness of Sinai, and they had no children: and Eleazar and Ithamar ministered in the priest’s office in the sight of Aaron their father.” 

Matthew 7:15-23 RSV
“Beware of false prophets, who come to you in sheep’s clothing but inwardly are ravenous wolves. You will know them by their fruits. Are grapes gathered from thorns, or figs from thistles? So, every sound tree bears good fruit; but the corrupt tree bears evil fruit. A good tree cannot bear evil fruit, nor can a corrupt tree bear good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire. Thus you will know them by their fruits.”

“Not every one who says to me, ‘Lord, Lord,’ shall enter the kingdom of heaven, but he who does the will of my Father who is in heaven. On that day many will say to me, ‘Lord, Lord, did we not prophesy in your name, and cast out demons in your name, and do many mighty works in your name?’ And then will I declare to them, ‘I never knew you; depart from me, you evil doers.”

Matthew 24:11, 24; Mark 13:22-23 RSV
"Many false prophets will arise and lead many astray…”

"For False Messiahs and false prophets will arise and show signs and wonders, to lead astray, if possible, even the elect. But take heed; I have told you all things beforehand."

2 Corinthians 11:12-15, 19-20 RSV + Amplf
“And what I do [says Paul, and so say I] I will continue to do, in order to undermine [and expose as false] the claim of those who would like to claim that in their boasted mission they work on the same terms as we do. For such men are false apostles, - spurious, counterfeits - deceitful workmen, disguising themselves, masquerading as apostles of Messiah. And no wonder, for even Satan disguises himself as an angel of light. So it is not strange if his servants also disguise themselves as servants of righteousness. Their end will correspond to their deeds.” 

“For you readily and gladly bear with the foolish, tolerating them, since you are so smart and wise yourselves. For you tolerate it if a man assumes control of your souls and enslaves you, or devours (your substance, spends your money) and preys upon you, or deceives and takes advantage of you, or is arrogant and puts on airs, exalting himself, or hits you in the face.”

2 Peter 2:1-2
“But false prophets also arose among the people, just as there will be false teachers among you, who will secretly bring in destructive heresies, even denying the Master who bought them, bringing upon themselves swift destruction. And many will follow their liscentiousness, and because of them the way of truth will be reviled.”

1 John 4:1
“Beloved, do not believe every spirit, but test the spirits to see whether they are of G-d; for many false prophets have gone out into the world.”

An expert diamond cutter, being intimately acquainted with the nature and appearance of a genuine diamond, quickly recognizes a counterfeit, sometimes even without any special tool. His eye is so trained to KNOW a genuine diamond that it instantly recognizes even the best of counterfeit. 

An expert on an artist’s paintings, let’s say Rubens, being intimately acquainted with every stroke of brush by that artist, without fail will recognize immediately even the finest of counterfeit, while those less acquainted with that specific artist will accept the counterfeit as the original and not discern it as being false. 

Likewise the lack of intimate knowledge of the LORD Who is Father, Son and Holy Ghost, and of His Word will cause many to accept men masquerading as apostles and prophets and evangelists as genuine servants of the Lord when indeed their claims are false, for they are imposters. 

G-d does not admonish us in vain that there is but one thing we may boast of, for He says: “Let not the wise man glory in his wisdom, let not the mighty man glory in his might, let not the rich man glory in his riches: but let him who glories glory in this, that he UNDERSTANDS and KNOWS Me, that I am the LORD who practices steadfast love, justice and righteousness in the earth; for in these things I delight, says the LORD.”¹