Rival Christianities in the New Testament

1 John 2:18-20 ESV
Children, it is the last hour, and as you have heard that antichrist is coming, so now many antichrists have come. Therefore we know that it is the last hour. They went out from us, but they were not of us; for if they had been of us, they would have continued with us. But they went out, that it might become plain that they all are not of us. But you have been anointed by the Holy One, and you all have knowledge....

2 Corinthians 11:12-15 ESV
What I am doing I will continue to do, in order to undermine 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, deceitful workmen, disguising themselves as apostles of Christ. And no wonder, for even Satan disguises himself as an angel of light. So it is no surprise if his servants, also, disguise themselves as servants of righteousness. Their end will correspond to their deeds.

Revelation 2:2-3 ESV
“‘I know your works, your toil and your patient endurance, and how you cannot bear with those who are evil, but have tested those who call themselves apostles and are not, and found them to be false. I know you are enduring patiently and bearing up for my name's sake, and you have not grown weary.’”

Revelation 2:14-16 ESV
“‘But I have a few things against you: you have some there who hold the teaching of Balaam, who taught Balak to put a stumbling block before the sons of Israel, so that they might eat food sacrificed to idols and practice sexual immorality. So also you have some who hold the teaching of the Nicolaitans. Therefore repent. If not, I will come to you soon and war against them with the sword of my mouth.’”

Revelation 2:20-25 ESV
“‘But I have this against you, that you tolerate that woman Jezebel, who calls herself a prophetess and is teaching and seducing my servants to practice sexual immorality and to eat food sacrificed to idols. I gave her time to repent, but she refuses to repent of her sexual immorality. Behold, I will throw her onto a sickbed, and those who commit adultery with her I will throw into great tribulation, unless they repent of her works, and I will strike her children dead. And all the churches will know that I am he who searches mind and heart, and I will give to each of you according to your works. But to the rest of you in Thyatira, who do not hold this teaching, who have not learned what some call the deep things of Satan, to you I say, I do not lay on you any other burden. Only hold fast what you have until I come.

Jude 3-4 ESV
Beloved, although I was very eager to write to you about our common salvation, I found it necessary to write appealing to you to contend for the faith that was once for all delivered to the saints. For certain people have crept in unnoticed who long ago were designated for this condemnation, ungodly people, who pervert the grace of our God into sensuality and deny our only Master and Lord, Jesus Christ.

1 Timothy 1:3-7 ESV
As I urged you when I was going to Macedonia, remain at Ephesus so that you may charge certain persons not to teach any different doctrine, nor to devote themselves to myths and endless genealogies, which promote speculations rather than the stewardship from God that is by faith. The aim of our charge is love that issues from a pure heart and a good conscience and a sincere faith. Certain persons, by swerving from these, have wandered away into vain discussion, desiring to be teachers of the law, without understanding either what they are saying or the things about which they make confident assertions.

2 Thessalonians 3:17 ESV
I, Paul, write this greeting with my own hand. This is the sign of genuineness in every letter of mine; it is the way I write.

Luke 1:1-4 ESV
Inasmuch as many have undertaken to compile a narrative of the things that have been accomplished among us, just as those who from the beginning were eyewitnesses and ministers of the word have delivered them to us, it seemed good to me also, having followed all things closely for some time past, to write an orderly account for you, most excellent Theophilus, that you may have certainty concerning the things you have been taught.