Jesus prayed for the unity of all believers:
I do not pray for these [my disciples] alone, but also for those who will believe in Me through their word; that they all may be one, as You, Father, are in Me, and I in You; that they also may be one in Us, that the world may believe that You sent Me. And the glory which You gave Me I have given them, that they may be one just as We are one: I in them, and You in Me; that they may be made perfect in one, and that the world may know that You have sent Me, and have loved them as You have loved Me. John 17:20–23
Notice that immediately prior to these words, Christ prayed:
Sanctify them by Your truth. Your word is truth. As You sent Me into the world, I also have sent them into the world. And for their sakes I sanctify Myself, that they also may be sanctified by the truth. John 17:17–19
The context of Jesus’ prayer for the unity of those who would believe in Him is the sanctifying truth of God’s word – the very word that the Apostles subsequently delivered to the Church, and through which we now believe in Christ.
Through that word, we are being made ‘perfect in one’ – the word of God itself creates Christian unity. And through that uniting and perfecting word, we know that the Father sent His Son to die in our place, that the Father loves us for the sake of His Son and puts His righteousness to our account. Through God’s word, those who believe it have confidence that:
having been justified by faith, we have peace with God through our Lord Jesus Christ, through whom also we have access by faith into this grace in which we stand, and rejoice in hope of the glory of God. Romans 5:1–2
God’s word, then, is essential to our Christian faith. We hunger for sound doctrine, because sound doctrine is nothing other than that word, faithfully delivered.
The Christian life depends upon sound doctrine. But is it harmful to Christian unity to rebuke false doctrine and separate from those who wilfully assist in its propagation? Some seem to think so. Yet Christians are nowhere in Scripture called to unite around false doctrine or practice, but to reject error and instead speak the truth in love to one another. We are to ‘contend earnestly for the faith which as once for all delivered to the saints’ (Jude 3).
Christian unity originates with the unity of the Spirit that we have in Christ through His word. Paul writes:
I, therefore, the prisoner of the Lord, beseech you to walk worthy of the calling with which you were called, with all lowliness and gentleness, with longsuffering, bearing with one another in love, endeavoring to keep the unity of the Spirit in the bond of peace. There is one body and one Spirit, just as you were called in one hope of your calling; one Lord, one faith, one baptism; one God and Father of all, who is above all, and through all, and in you all. Ephesians 4:1–6
Notice that, even as he exhorts the Ephesians to keep the unity of the Spirit, he roots that unity in ‘one body and one Spirit’, ‘one hope of your calling’, ‘one Lord, one faith, one baptism, one God and Father of all’. These are the things of which sound doctrine speaks – ‘the faith which was once for all delivered to the saints’.
It is immediately obvious that false doctrine – that which is contrary to the faith once delivered – is necessarily destructive of Christian unity, for it seeks to lure people away after other bodies, other spirits, other hopes, other lords, other baptisms, other gods.
Paul goes on to tie the ‘unity of faith’ with ‘the knowledge of the Son of God’, explaining how both develop as God’s appointed ministers of the word equip the ‘saints for the work of the ministry’:
And He Himself gave some to be apostles, some prophets, some evangelists, and some pastors and teachers, or the equipping of the saints for the work of ministry, for the edifying of the body of Christ Ephesians 4:11–12
We have the word of Christ delivered through the Apostles and prophets of old. That same word, once delivered, is even now proclaimed, explained and applied by evangelists, pastor and teachers. These ministers of the word are charged with equipping the flock by that word until:
we all come to the unity of the faith and of the knowledge of the Son of God, to a perfect man, to the measure of the stature of the fullness of Christ; Ephesians 4:13
God has charged them with this task so that:
we should no longer be children, tossed to and fro and carried about with every wind of doctrine, by the trickery of men, in the cunning craftiness of deceitful plotting, but, speaking the truth in love, may grow up in all things into Him who is the head—Christ—from whom the whole body, joined and knit together by what every joint supplies, according to the effective working by which every part does its share, causes growth of the body for the edifying of itself in love. Ephesians 4:14–16
In other words, Christian unity develops as sound teaching overcomes the winds of false doctrine, and Christians mature together into Christ by the speaking of truth (and, contextually, the true doctrine of God’s word) in love.
Sound doctrine gives life. False doctrine, though, is the enemy of true Christian unity, the enemy of the Faith, and the enemy of our souls. Unity of faith cannot arise where false doctrine is tolerated.
A little (bad) leaven leavens the whole lump, making it unfit for consumption (Matt. 16:5–12; 1 Cor. 5.6; Gal 5:9). Likewise, water from even the purest well is rendered deadly by even a tiny amount of poison. So it is with false doctrine. That is why those who understand these things are ‘quick to argue theology’.
Hold fast the pattern of sound words which you have heard from me, in faith and love which are in Christ Jesus. That good thing which was committed to you, keep by the Holy Spirit who dwells in us. 2 Timothy 1:13–14