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The apostles fixed forever both the doctrine and the Gospel (good news). The doctrine is complete.
Acts 2:42 And they continued steadfastly in the apostles’ doctrine and fellowship, in the breaking of bread, and in prayers.
1 Timothy 1:3 As I urged you when I went into Macedonia—remain in Ephesus that you may charge some that they teach no other doctrine,
1Timothy 4:16 Take heed to yourself and to the doctrine. Continue in them, for in doing this you will save both yourself and those who hear you.
2John 1:10 If anyone comes to you and does not bring this doctrine, do not receive him into your house nor greet him;
The gospel is complete.
Mark 16:15 And He said to them, "Go into all the world and preach the gospel to every creature. 16 "He who believes and is baptized will be saved; but he who does not believe will be condemned.
Galatians 1:8 But even if we, or an angel from heaven, preach any other gospel to you than what we have preached to you, let him be accursed.9 As we have said before, so now I say again, if anyone preaches any other gospel to you than what you have received, let him be accursed.
All changes that have arisen since the end of the 1st century have been corruptions. Corrupt doctrines include the Trinity, immortal souls, division into priesthood and laity, heaven-going at death, the church is the kingdom of God, hell as a place of eternal torment, ‘just’ wars etc.
The Bible says that unity depends on agreement.
Amos 3:3 Can two walk together, unless they are agreed?
There can only be unity if they that preach carry the same Gospel. The penalty for preaching a different Gospel to that preached by Paul is severe. The departure of Christianity from the truth is both prophesied and a blindingly obvious fact of history.
2 Timothy 4:3 For the time will come when they will not endure sound doctrine, but according to their own desires, because they have itching ears, they will heap up for themselves teachers; 4 and they will turn their ears away from the truth, and be turned aside to fables.
The scriptures themselves are able to make us ‘wise unto salvation’.
2Timothy 3:15 and that from childhood you have known the Holy Scriptures, which are able to make you wise for salvation through faith which is in Christ Jesus.
The Bible is the standard against which the doctrines and gospels of the different churches can be checked. For this reason we urge everyone to read the Bible for themselves and not be mislead by apostate ‘Christian’ church preaching which will not save.
Unity comes from belief of the ‘truth’. There is, and can be only, one ‘truth’.
I hope this helps.