When Jesus prayed for His redeemed to be one with Him and the Father, He didn't say this was to be done by 'loving one another' or by believing in the same doctrine. He said, quote...John 17:9 I pray for them: I pray not for the world, but for them which thou hast given me; for they are thine.....11 Holy Father, keep through thine own name those whom thou hast given me, that they may be one, as we are. ......
Now of course they believed the same doctrine, and they loved one another. However I do not think this was the primary reason there were 'one'.
21 That they all may be one; as thou, Father, art in me, and I in thee, that they also may be one in us: that the world may believe that thou hast sent me.
22 And the glory which thou gavest me I have given them; that they may be one, even as we are one:
I believe the above bolded part is the key to unity. As the Father was in His Son, so also may we be in them.
As we abide in Christ...as we grow closer to Him, relationally, then we shall grow closer to the Father. Think of two parallel lines fading off into the distant horizon, you on one, me on another. We are a mountain range apart on doctrine, and though we discuss our various perspectives it seems we never come to a unity or agreement. Yet we are both drawing closer to Christ and the Father. In Him we are converging and coming closer to one another. As we discuss and debate, as we study and pray, the Spirit works in each of us, the Word creates in us the righteousness of Christ, and we two become more and more like Christ in character, goals, and purpose. The doctrines will take care of themselves as Christ teaches us both. The mountain range will turn into a hill, then a plain, yet in Him we are still one.
But seeking unity in any shape or form whether through love or doctrine, is an utter waste of time if we are not in Christ. If we are in Him, if we are abiding in Him, if we remain in Him, there will come a time when we will be teaching and believing and practicing the same doctrine. Practising sound doctrine is the inevitable fruit of knowing our Saviour. We are sanctified by Truth...sound doctrine...as found in scripture. A limited knowledge of scripture is sufficient for all to come to a place of salvation or justification. We may not know what justification means, but we may be born again without understanding all scripture perfectly. But as we continue to abide in the Vine, the sap and strength of the Vine will give us growth and understanding.
When Jesus comes to take His people home, not everyone will be in total agreement on doctrine. But they will be filled with His holy Spirit...they will have His character, and as has been alluded to previously, the fruits of that Spirit is love. The remaining 'fruit' as revealed in Galatians being joy, peace, longsuffering, gentleness, goodness, faith, meekness and self control, are descriptives of the love we are to have. It is God's nature...Christ's likeness within, that will bring us in union with one another. This is affected by being in Him and in the Father, just as they are in one another.
First, abide. Then be sanctified by Truth, filled with His Spirit, love will result, unity the end goal. But not all will be found in Him, despite their profession. There cold hearts revealing this to the church, and to the world.