Please permit a brief excerpt from my book on this subject that may prove helpful:
CHAPTER 1:
THE CHURCH IS ONE, BECAUSE
GOD IS ONE
God Is the Ultimate Unity
Foundational to the understanding of the nature of the Church is the nature of God Himself. To say it another way, the Church can and should only be a reflection of God Himself. Therefore, let us see what Scripture makes clear about His nature and apply His nature to His Church:
“Hear, O Israel: The LORD our God, the LORD is one." (Deuteronomy 6:4 ESV)
The oneness of God is not in any way denigrated by the fact that this One God is named Father, Son, and Holy Spirit. (Matthew 28:19; II Corinthians 13:14; Hebrews 9:14; Matthew 3:16-17)
Human reason blinks with a lack of understanding at the Triune nature of the One true God because human reason simply cannot adequately comprehend what God has revealed Himself to be. Neither can man adequately understand the profundity of God's unity for it too is quite beyond any human experience of unity. Thus, God must simply be listened to, and His Words must stand and be believed as they are given us. Those who try too hard to apply human reason to either the Triune nature of God or His surpassing unity will only end up replicating the errors of the early Church Christological and Trinitarian controversies that divided the early church for more than four centuries. This human conjecture only results in shrinking God down to an inadequate human size. To go farther than what God reveals of Himself in His Word is only to enter the world of subjective human speculation. Such a journey is without benefit and inevitably leads away from orthodox (true) teaching to heterodox (false) teaching.
As Dr. Francis Pieper, renowned Lutheran dogmatician and the fourth President of The Lutheran Church-Missouri Synod, wrote:
"...He teaches us the existence in the Divine Majesty of the one undivided essence, but this in such manner that there is, first, the Person who is called the Father; and of Him the Second Person called the Son, born from eternity; and proceeding from both of these is the Third, namely, the Holy Ghost. These three Persons are not of separate essence from each other, as individual brothers or sisters are, but they have one and the same eternal, undivided, and indivisible essence in common."
I would draw your attention particularly to the use of the terms "undivided" and "indivisible" in Pieper's quote above. Although mere human words cannot adequately capture the oneness or unity of God, we find such words helpful toward that end. These words tell us that there can be no hint of division nor lack of unity within the God-head. Again, this very thought is echoed by our Lord Himself in His high priestly prayer in the Gospel of St. John when He says:
“I do not ask for these [the apostles] only, but also for those who will believe in me through their word, that they may all be one, just as you, Father, are in me, and I in you, that they also may be in us, so that the world may believe that you have sent me. The glory that you have given me I have given to them, that they may be one even as we are one, I in them and you in me, that they may become perfectly one, so that the world may know that you sent me and loved them even as you loved me. Father, I desire that they also, whom you have given me, may be with me where I am, to see my glory that you have given me because you loved me before the foundation of the world. O righteous Father, even though the world does not know you, I know you, and these know that you have sent me. I made known to them your name, and I will continue to make it known, that the love with which you have loved me may be in them, and I in them.” (John 17:20-26 ESV)
God Created The Church To Be
In Unity With Him
In this key passage from John 17, Jesus clearly and repeatedly indicates that His Church (inclusive of the Church visible on earth) is to have the same quality of oneness and unity that He (the Son) has with the Father. Twice, once in verse 21 and again in verse 23, Jesus specifically says that this unity is to exist within His Church so that, "the world may believe that you sent me", and, "...that the world may know that you have sent me...". So this is not a description of the Church only in heaven, but rather also the Church visible on this planet. Notice also that it is our Lord's desire, "...that they may become perfectly one...". (emphasis added) This leaves no wiggle room respecting the degree of the oneness that Jesus desires for His Church. When our Lord says “perfect”, He actually means perfect.
So in this way, our Lord describes a direct parallel between the unity of the God-head and the unity that should mark His Church on earth. Therefore, the Church on earth is one precisely so it stands in stark contrast to the Satanic divisions of our fallen world, and hence the world will know that the Father has sent the Son. If, therefore, God is One, so then also must His Church visible be one!
God Reveals Himself With Complete Unity
But in what sense is this oneness made manifest? Is it merely that all who believe in Christ's sacrifice and merits for their salvation makes us one? Or is there something more here? Let's see how the logic plays out in this question.
Would the Church of our Lord be truly one if there are factions within the Church that teach contradictory doctrines or practice those doctrines in unbiblical ways? Doesn't false teaching/doctrine always divide the Church? Isn't any false doctrine destructive to the unity/oneness of the Church? Would such false teaching and/or unbiblical practice still be divisive even if someone incongruously also believes in Jesus as their Savior from sin?
The answer to each of the above questions is, of course, clear. This is the case precisely because each and every doctrine and its practice must be a reflection of the very nature of God Himself, and in Him there can never be anything false or even questionable.
As St. John wrote in his first epistle:
"This is the message we have heard from him and proclaim to you, that God is light, and in him is no darkness at all. If we say we have fellowship with him while we walk in darkness, we lie and do not practice the truth. But if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus his Son cleanses us from all sin. If we say we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness. If we say we have not sinned, we make him a liar, and his word is not in us." (I John 1:5-10 ESV)
Certainly false doctrine and unbiblical practice are manifestations of "darkness". They most certainly are not light. If there can be no darkness in God at all, then neither can His Church participate in darkness for that would be having fellowship of light with darkness. To have fellowship with God is to walk in the light, which is the absence of darkness. Thus, to profess faith in God and union with God but at the same time to embrace false teaching - any false teaching - is to make God out to be a liar, and this the true Church should never do!
Permit then, the obvious application: The various denominations, synods, and associations of congregations are not all on the same side. Once again, it was precisely this truth that the Lutheran Confessions make in the Preface to the Augsburg Confession:
"After the removal and correction of things that either side has understood differently, these matters may be settled and brought back to one simple truth and Christian concord. Then we may embrace and maintain the future of one pure and true religion under one Christ, doing battle under Him [Psalm 24:8], living in unity and concord in the one Christian Church."
Here it is more than clear that there is only, "...one Christian Church." and there is only one "...pure and true religion...". That is the nature of God, and it is the nature of His Church. Thus, the Lutheran Confessions never make a distinction between the Christian faith and the Lutheran faith but consider the terms to be synonymous. If there are differences in doctrine, then someone is wrong, and that which is, "...understood differently" must be "settled and brought back to one simple truth and Christian concord.", or there is no unity. Instead of being the Christian Church, such an errant institutional body becomes a Christian "sect" which holds to some truth and some falsehood. Again, the true Christian Church visible cannot and will not tolerate nor embrace error at all. To do so is to violate the very nature of God who has called His Church into being and whose Son died for her. To do so is to forfeit being the Church and to become instead, a Christian sect.
To help us understand what is at stake respecting whether or not the unity of the true, Church visible on earth can possibly hold to some false teaching, consider some definitions and distinctions that are helpful:
Church: Where the Word of God is taught in all its truth and purity, and where the Sacraments are administered in accord with Christ's institution. Lutherans have always believed, taught, and confessed that there is no distinction between the Christian Church and the faith of orthodox Evangelical Lutheranism.
Sect: A congregation or institutional church body that understands the saving grace of God for sinners, yet inconsistently holds to, or tolerates without correction, one or more false teachings.
Cult: A congregation or institutional body that may give the appearance of being Christian, but does not believe, teach, or confess that sinners are justified before God through the life, suffering, death, and merits of Christ alone. If someone actually believes in this unChristian theology, they cannot be saved.
While it is certainly possible for a congregation or an institutional church body to understand the Gospel, and yet inconsistently hold to one or more false teachings, such a Christian sect cannot claim to be the Church. Why? Because it does not teach the Word of God in all its truth and purity. Thus, such a congregation, or institutional church body, is not merely another kind of Christian Church, but forfeits the right to be the Church and has become a Christian sect. Who are the sects? Any congregation or institutional church body that does not teach the Word of God in all its truth and purity and which does not administer the Sacraments of Holy Baptism and the Lord's Supper as Christ instituted them.
Those who belong to a cult and actually believe what their cult believes, teaches, and confesses, are not part of the Christian Church no matter how much they may think they are. It is possible, however, for a cult member to believe something different than that which is taught by the cult, and in such cases a person might be saved because of the promises of Christ that apply to them. Examples of pseudo-Christian cults include but are not limited to; The Church of Jesus Christ of Latter Day Saints, Jehovah's Witnesses, the Unitarian Universalist Church, and the Unification Church.
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