The expression in this title comes from Ephesians 4:12-13. Paul lists some of the offices in the early church and then provides the reason why Christ made these appointments: “God’s consecrated people should be fully equipped, that the work of service might go on, and that the body of Christ should be built up. And this is to go on until we all arrive at complete unity in faith in and knowledge of God, until we reach perfect manhood, until we reach a stature which can be measured by the fullness of Christ” (Barclay’s translation).
The expression “fully equipped” (Greek katartizein) is one which was used in politics for bringing together opposing factions so that government could go on. It was used in surgery for setting a broken limb or putting a joint back into its place, and in the NT for mending nets (Mark 1:19). In other words, the term is one which is used for “putting a thing into the condition in which it ought to be”.
Paul then uses an expression, accurately translated by Barclay as ‘arrive at’, which means to arrive at a destination. He is making it clear that this is a primary goal or ‘mission’ of the church. It is a threefold destination:
1. Unity in faith;
2. Perfect manhood;
3. A stature which can be measured by the fullness of Christ.
We will look at some of these terms in more detail.
UNITY IN FAITH
To some people this expression describes an agreement with tenets of religion, a common creed or Statement of Faith. However, the object of a creed or statement of faith is usually to define who IS in the group as well as who IS NOT. For example, the Birmingham amended Statement of Faith defines ‘the faith’ both “positively and negatively” with 31 clauses stating what the ecclesia does believe, and 35 “doctrines to be rejected”, or what it does not believe. The fact that the BASF has been amended betrays the historical reality that the original definition of ‘the faith’ had to be changed in order to exclude from fellowship those who differed with its authors on matters not already covered. Hence it is exclusive rather than unifying.
I believe Barclay has structured his translation of this verse correctly when he speaks of “unity in faith in and knowledge of God”. It is our faith, or trust in God and our personal knowledge of Him which unites us, not our acceptance of a summary of doctrines. Francis Foulkes got it right when he wrote: “Faith is not just the acceptance of a collection of dogmas, in the embracing of which unity will be found. It is something deeper and more personal. It is unity in the knowledge of the Son of God. We can never know any person simply with our mind; and knowledge of such a Person as is envisaged here must involve the deepest possible fellowship.” 
It seems that the early Christadelphians weren’t as exclusive in their fellowship as later generations. John Thomas, for example, continued to meet and fellowship with congregations belonging to the Disciples of Christ or Churches of Christ, even after renouncing his earlier beliefs and the teachings of the leaders of that denomination and being re-baptised. The first Constitution of what could be called a Christadelphian ecclesia was of the Royal Association of Believers in New York, of which brother Thomas was a member, in 1854 (the name ‘Christadelphian’ not being coined until 1864). The clause on ‘fellowship’ stated:
“Being the Lord’s table, and not the table of the Association, all of good report within the city or without it, who believing the gospel of the kingdom, have been immersed, are cordially invited to worship with us; the only privileges withheld being a participation in the direction of our affairs, and speech without previous invitation.” 
In this early Constitution fellowship, or “the privilege of Christian worship”, was distinguished from “membership” of the Association. Fellowship was open to “all of good report” who believed the gospel and had been immersed. No attempt was made to restrict fellowship to members only, on the grounds that it was “the Lord’s table, and not the table of the Association”. Membership was disallowed “to any immersed believers who cannot prove that they walk as becomes the kingdom of God and of Christ.”
In the case of both fellowship and membership the emphasis was on the need to have a good reputation and to live a lifestyle consistent with the Gospel. How times have changed when the emphasis of interviews for baptism or membership of an ecclesia is on what a person believes rather than on how they live! And what of those ecclesias who welcome visitors at the door with a card stating their doctrinal or fellowship position and asking for an endorsement before the visitor can proceed further! Have they forgotten that it is the Lord’s table and not their own?
The Association is defined as "an aggregation of persons who believe 'the things' covenanted to Abraham and to David, 'concerning the kingdom of God and the Name of Jesus Christ', and have therefore been 'immersed into the name of the Father, and of the Son, and of the Holy Spirit'." No further definition of the Gospel is included or appended. What constituted the Gospel was apparently understood by the members without having to be enshrined in a creed or Statement of Faith.
The expression “unto a perfect man” describes a mature adult. The Greek word translated perfect (teleios) means to be mature. The word ‘man’ describes an adult as distinct from a child (rather than a man as distinct from a woman) and is used to express this distinction in 1 Corinthians 13:11. It is necessary for the church to go through a process of growth and maturing. Divisions, quarrels and disunity are evidence of spiritual immaturity and childishness.
Growth, of necessity, means change. As we mature we inevitably change and adapt our thinking and attitudes. Some brethren, however, see their refusal to change one iota from the ideas and expressions of a previous generation as a strength. They see it as a sign of consistency. Having said “I still teach what I have taught for years” John Thomas, however, readily admitted that as he “learned from the word” he had changed his thinking on many matters. “Must a man never progress? If he discovers an error in his premises, must he for ever hold to it for the sake of consistency? May such a calamity never befall me! Rather let me change every day, till I get right at last.” 
THE MEASURE OF THE STATURE OF THE FULNESS OF CHRIST
The word translated ‘stature’ means age or physical stature and is used here figuratively of maturity. Our spiritual maturity can be measured by “the fulness of Christ”. Paul uses this same expression in 1:23 and Colossians 1:10 to explain that the church should be the full expression of Jesus Christ by being filled by him.
The context of this passage shows that the church is filled by Christ by making full use of the gifts and grace that he seeks to impart. Paul’s language here, including his repeated allusions to the human body, is similar to that in 1 Corinthians 12 where he describes one body with many parts. There he argues that “God has arranged the parts in the body, every one of them, just as he wanted them to be” (v 18), and goes on to say that “there should be no division in the body, but that its parts should have equal concern for each other” (v 25). He concludes with a list of ‘office-bearers’ (v 28ff), similar to that in Ephesians, to emphasise that we all have different abilities and different jobs to do but that the same God is working in each one for the common good (see v 6f).
Now both these Scriptures are emphasising to us that we must value the contribution each brother and sister can make. Regardless of how different our abilities, or level of personal maturity or knowledge, we must recognise that God is working with and in each individual for the common good. Hence no part of the body can afford to be without another , or to cut itself off from the rest of the body (’The eye cannot say to the hand, “I don’t need you!”’ v21). Yet isn’t that precisely what we attempt to do every time we ‘disfellowship’ an individual or an ecclesia, or group ourselves into ‘fellowships’?
Paul concludes his appeal to the Ephesians for “unity of the Spirit through the bond of peace” (4:3) with another allusion to the human body. “Speaking the truth in love, we will in all things grow up into him who is the Head, that is, Christ. From him the whole body, joined and held together by every supporting ligament, grows and builds itself up in love, as each part does its work” (v 15f). His expression “held together” translates the Greek word sunbibazomenon, which is used of bringing things or people together, and of reconciling those who have quarrelled . Like the supporting ligaments which hold a joint together and enable it to function correctly, so love enables those who have quarrelled to be reconciled. Has this been the missing ingredient in previous fellowship discussions?
Note that Paul stresses both here and earlier that the body depends for its growth on the Lord’s direction and arrangements. We can grow and prosper only to the extent that we follow his leadership and are in tune with him.
 William Barclay, The Letters to the Galatians and Ephesians, The Saint Andrew Press, Edinburgh, 1976, p.145.
 Ibid, p.149.
 Francis Foulkes, The Epistle of Paul to the Ephesians, Wm.B.Eerdmans, Grand Rapids, Michigan, 1956, p121.
 John Thomas “Constitution of the Royal Association of Believers in New York” Herald of the Kingdom and Age to Come, January, 1854.
 From a letter written by John Thomas in 1848, quoted by Robert Roberts, Dr. Thomas: His Life and Work, The Christadelphian, Birmingham, 1954, p.153 (his emphasis).
 Foulkes, op cit, p.124.