I say this in the spirit of the Exsultet.
O happy fault,
O necessary sin of Adam,
which gained for us so great a Redeemer!
Currently I am reading Blessed John Henry Newman’s “An Essay on the Development of Christian Doctrine” and was thinking how much we owe to heretics.
Much of the Old Testament with the Judges, Prophets, and Chronicles show the ebb and flow of the faithfulness of the people of Israel. The Prophets were not exactly called to remind people what a good job they were doing worshiping God. Again and again the prophets called people back to true worship and had plenty of choice words about their current behavior. A faithful Israel would have made for a much shorter Old Testament.
The New Testament is much the same. Outside of the Gospels and the Books of Acts most of the letters address problems in the new Christian communities from liturgical abuses to scandals of members involved in sin. Again, faithful communities would have brought us a much shorter New Testament.
Moving in to the age of the Church things haven’t changed much. The development of doctrine owes much to heretics. Blessed John Henry Newman notes the two aspects of doctrinal development 1. Investigations of Faith, 2. Attacks of heresy. The major Ecumenical Councils of the Church were mostly called in response to attacks of heresy. This seems especially true for the Councils up through Trent. As a result of Arius we got a much more defined Christology and this has been the pattern. The Church is much more likely to respond to heresies and seriously disputed questions as the needs arise. On other theological questions she is willing to wait for the turn of centuries and for the investigations of faith to more fully elucidate some truth. For example the Immaculate Conception had been believed since the beginning of the Church. The theology involved was advanced by Blessed Duns Scotus in the 13th century and yet the Church waited till 8 December 1854 to have Pope Pius IX formally define it. The issue of providence of God with human free will was a theological battleground between Molinism and Thomism enough so that the pope of the time basically called a time out. The issue has been pretty much left on hold as far as doctrinal development goes. Again the Church is willing to let centuries and millenniums pass when it comes to doctrinal development and is quite willing to take her cues from the Holy Spirit in this regard.
Heretics get results and so thanks to heretics we get doctrinal action. So thank you heretics, I just hope you repented before you died.