This is an interesting question, and it assumes that the Church did stand in apostasy for 1500 years. However, that is not what Lutherans believe about Luther or about the Church prior to Luther. Since the Church is filled with sinners, it is always in need of reformation, not only in the Sixteenth Century, but also in the Second, Fifth, Ninth, and Twenty-first centuries. At His Ascension, Jesus did indeed promise to be with His Church always, and as the Augsburg Confession puts it, “they [the Lutheran theologians] teach that one holy church will remain forever. The Church is the assembly of saints in which the gospel is taught purely and the sacraments are administered rightly” (Kolb/Wengert 43, VII:1 [my emphasis]; also here). Even in the midst of medieval errors and confusion, wherever the Lord’s Words in the Sacrament of the Altar were still heard or the Gospel of the free forgiveness was preached, the Holy Spirit was still working. Otherwise, Luther as the reformer never could have existed. There never was a pure Church, even when the Apostles were alive, in the sense that it did not need to be reformed by repentance and the continuing Life of Christ in His Word and Sacraments (the Book of Acts, 1 Corinthians, and Galatians provide ample evidence of this).
Luther and the other reformers were clear: whatever they preached had to be the same Gospel preached by the Apostles from Jesus Himself, and handed down through the centuries, even when it was clouded by error. They confessed the same pure teaching confessed by other teachers formed by the Scriptures. They demonstrated this by including a section after the Formula of Concord called the Catalog of Testimonies, filled with quotations of earlier Fathers of the Church on the various issues addressed in the Book of Concord. (You can read it here.)
We don’t believe the Church existed (or exists today) because we can see it and how pure it is. We believe it now, as Luther did then, because Jesus’ promise cannot fail. He was with the Church through the first 1500 years of its existence, just as He has been for the past 500. That is why we continue to confess, along with the Church of all times and places, “I believe in one, holy, catholic, and apostolic Church.” Like Luther and the other reformers, we walk by faith and not by sight, trusting the pure teaching of Jesus, which He delivered once and for all to the saints.
Pr. Timothy Winterstein