Theology of grace
Martin Luther argued that because the Catholic church had "lost sight of the doctrine of grace", it had "lost its claim to be considered as the authenthic Christian church."; this argument was open to the counter-criticism from Catholics that he was thus guilty of schism and a Donatist position, and in both cases therefore opposing central teachings of Augustine of Hippo.
Against denominationalism and schism
Yet Luther, at least as late as 1519, argued against denominationalism and schism, and the Augsburg Confession of 1530 can be interpreted (e.g. by McGrath 1998) as conciliatory  (others, e.g. Rasmussen and Thomassen 2007 argue convincingly with evidence that Augsburg was not conciliatory but clearly impossible for the Roman Catholic Church to accept ). "Luther's early views on the nature of the church reflect his emphasis on the Word of God: the Word of God goes forth conquering, and wherever it conquers and gains true obedience to God is the church":
Ecclesia sancta catholica
"Now, anywhere you hear of see such a word preached, believed, confessed, and acted upon, do not doubt that the true ecclesia sancta catholica, a 'holy Christian people' must be there...." "Luther's understanding of the church is thus functional, rather than historical: what legitimates a church or its office-bearers is not historical continuity with the apostolic church, but theological continuity." 
John Calvin is among those working, primarily after Martin Luther, in the second generation of Reformers, to develop a more systematic doctrine of the church (i.e. ecclesiology) in the face of the emerging reality of a split with the Catholic church, with the failure of the ecumenical Colloquy of Regensburg in 1541, and the Council of Trent's condemnation in 1545 of "the leading ideas of Protestantism". Thus, Calvin's ecclesiology is progressively more systematic.
Emphasis on predication
The second edition of Calvin's Institutes of the Christian Religion in 1539 holds that "the marks of the true church [are] that the Word of God should be preached, and that the sacraments be rightly administered". Later, Calvin developed the theory of the fourfold office of pastor, doctor (or teacher), elder, and deacon, possibly owing to the colleagueship with Martin Bucer and his own experience of leadership in church communities.
Visible and invisible church
Calvin also discusses the visible church and the invisible church; the visible church is the community of Christian believers; the invisible church is the fellowship of saints and the company of the elect; both must be honoured; "there is only one church, a single entity with Jesus Christ as its head" (McGrath); the visible church will include the good and the evil, a teaching found in the patristic tradition of Augustine and rooted in the divine teaching, recorded in the Gospel according to Matthew, of the Parable of the Tares(Mt 13:24-31); thus, Calvin held that it is "not the quality of its members, but the presence of the authorised means of grace, [that] constitutes a true church" (McGrath).
Concerns about fragmentation
Calvin was concerned to avoid further fragmentation, i.e. splits among the Evangelical churches: "I am saying that we should not desert a church on account of some minor disagreement, if it upholds sound doctrine over the essentials of piety, and maintains the use of the sacraments established by the Lord."
Radical Reformation ecclesiology
There is no single "Radical Reformation Ecclesiology". A variety of views is expressed among the various "Radical Reformation" participants.
- They verily are not the true congregation of Christ who merely boast of his name. But they are the true congregation of Christ who are truly converted, who are born from above of God, who are of a regenerate mind by the operation of the Holy Spirit through the hearing of the divine Word, and have become the children of God, have entered into obedience to him, and live unblamably in his holy commandments, and according to his holy will with all their days, or from the moment of their call.
This was in direct contrast to the hierarchical, sacramental ecclesiology that characterized the incumbent Roman Catholic tradition as well as the new Lutheran and other prominent Protestant movements of the Reformation.
Some other Radical Reformation ecclesiology holds that "the true church [is] in heaven, and no institution of any kind on earth merit[s] the name 'church of God.'"
- McGrath, Alistair. 1998. Historical Theology, An Introduction to the History of Christian Thought. Oxford: Blackwell Publishers. p. 200.
- McGrath, op.cit. p.201
- Rasmussen, Tarald, and Thomassen, Einar. 2007. Kristendomen: en historisk introduktion. Artos and Norma Bokförlag. p.294
- McGrath. op.cit. p. 202.
- Luther, M. 1539. On the Councils and the Church. In, D. Martin Luthers Werke: Kritische Gesamtausgabe. vol. 50. Weimar: Böhlau (1914). 628.29-630.2.
- McGrath. op.cit. p.202.
- McGrath. op.cit. pp. 200-7.
- McGrath. op.cit. p.205.
- McGrath. op.cit. pp. 205-6.
- McGrath. op.cit. p. 206.
- Calvin, John. Institutes, IV.i.9-10. In, Joannis Calvini: Opera Selecta. Vol. 5. Ed., P. Barth and W. Niesel. Munich: Kaiser Verlag, 1936. 13.24-16.31. Citation and information from McGrath. op.cit., pp. 205-7.
- George, Timothy (1988). Theology of the Reformers. Nashville, Tennessee: Broadman Press. p. 285.
- McGrath, op.cit. p. 200.
- Jesus and His Church. A study of the idea of the Ecclesia in the New Testament... (London: Epworth Press, 1938; last reprint: Carlisle: Paternoster, 1998). Fernley-Hartley lecture.