The Church is universal. Pieper explains that it is composed of all people, of all eras and places, who had faith in the remission of sins because of Christ's vicarious atonement. This is not something invented by the Reformation fathers. Paul uses the Old Testament scriptures to prove justification by faith without works. He says in Galatians 3 that all who believe it are, like those who believed likewise before Christ, Abraham's seed.
Pieper also says that there can be, and are, Christians in heterodox churches. The thing that makes a man a Christian is believing that God is gracious and merciful to him because of the vicarious atonement of Christ. To say otherwise is to deny the doctrine of Justification (Pieper, 1953). Such a situation is certainly not, however, ideal for the Christian who finds himself in it.