Eph 4: 4-6 "One body there is, and one spirit, even as you were called in the one hope to which you were called; one Lord, one faith, one baptism; one God and Father of all [persons], who is over all the through all and in all."

Eph 4:13-14 "until we all attain to the oneness in the faith and in the accurate knowledge of the Son of God, to a full-grown man, to the measure of stature that belongs to the fullness of the Christ; in order that we should no longer be babes, tossed about as by waves and carried hither and thither by every wind of teaching by means of the trickery of men, by means of cunning in contriving error."

Mt 7:13-23 "Go in through the narrow gate; because broad and spacious is the road leading off into destruction, and many are the ones going in through it; whereas narrow is the gate and cramped the road leading off into life, and few are the ones finding it.
“Be on the watch for the false prophets that come to you in sheep’s clothing, but inside they are ravenous wolves. By their fruits you will recognize them. Never do people gather grapes from thorns or figs from thistles, do they? Likewise, every good tree produces fine fruit, but every rotten tree produces worthless fruit; a good tree cannot bear worthless fruit. Every tree not producing fine fruit gets cut down and thrown into the fire. Really, then, by their fruits YOU will recognize those [men].”
“Not everyone saying to me, ‘Lord, Lord,’ will enter into the kingdom of the heavens, but the one doing the will of my Father who is in the heavens will. Many will say to me in that day, ‘Lord, Lord,’ did we not prophesy in your name, and expel demons in your name, and perform many powerful works in your name?’ And yet then I will confess to them: I never knew YOU! Get away from me, YOU workers of lawlessness.”

Mt 16:6, 12 “Jesus said to them: “Keep YOUR eyes open and watch out for the leaven of the Pharisees and Sadducees….Then they grasped that he said to watch out, not for the leaven of the loaves, but for the teaching of the Pharisees and Sadducees.”

Ga 5:7 “YOU were running well. Who hindered you from keeping on obeying the truth? This sort of persuasion is not from the One calling YOU. A little leaven ferments the whole lump.”

2Ti 3:5-7 “…having a form of godly devotion, but proving false to its power; and from these turn away. For from these arise those men who slyly work their way into households and lead as their captives…always learning and yet never able to come to an accurate knowledge of truth.”

2Cor 6:14-17 “Do not become unevenly yoked with unbelievers. For what fellowship do righteousness and lawlessness have? Or what sharing does light have with darkness? Further, what harmony is there between Christ and Belial? Or what portion does a faithful have with an unbeliever? And what agreement does God’s temple have with idols? For we are a temple of a living God; just as God said: “I shall reside among them and walk among [them], and I shall be their God and they will be my people.” “ ‘Therefore get out from among them, and separate yourselves,’ Says God, ‘and quit touching the unclean thing’”, “ ‘and I will take YOU in.’ ”

Re 18:4 “And I heard another voice out of heaven say: “Get out of her, my people, if YOU do not want to receive part of her plagues. For her sins have massed together clear up to heaven, and God has called her acts of injustice to mind.”

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These scriptures do not apply just to faiths other than Christianity. For within Christendom, there are many, many sects that separates us. Even popular ones! Do I believe there are false teachings among Christendom? You betcha!

If we are already divided by our “same teachings,” how could we possibly consider being of “one accord?” We cannot. One must discern whether or not their faith contradicts itself through its doctrines/interpretations, and its works v. Biblical teachings. If there is hypocrisy and contradiction, it is not “producing fine fruit,” then it is false teaching (and God taught us how to recognize this).

I can think of three main sects in Christendom in North America just off the top of my head that rely more on their own doctrines and other books then that of the Bible. Would I choose to follow those teachings? NO. Do they call themselves Christian? Yes, they do.