At the radical end of the Protestant Reformation of the early C16, the Mennonites (so-called) were part of a wider movement seeking a return not only to a Biblical doctrine of salvation but were also prepared to look more closely at the implications of this salvation for an understanding of the nature of the church.
The idea of a local church as a community of saved believers is so widely accepted now that it is difficult to imagine how radical a concept this was in the context of C16 Europe.
While Luther was articulating a clear doctrine of justification through faith, and could see how clearly this truth challenged the Catholic concepts of the mass, the priesthood, indulgences, purgatory and the veneration of Mary and the saints, Luther remained firmly wedded to an institutional model of the church. When his followers were forced to separate from the Catholic structures, it was reluctantly and with the aim of establishing a purified state church, a mixture of saints and sinners.
Groups such as the Mennonites went much further than this. Seeing clearly that those who had been justified through faith comprised the people of God on the earth, called out and separate from the world, these Anabaptist groups began to understand the church as a community of believers committed to one another and to a life of discipleship.
Although numerically small, the influence of these groups continues to be felt today as modern evangelicalism now broadly accepts the concept of a church made up of believers. Whether contemporary evangelicalism takes as seriously the related call of a life of discipleship is another question.