Luther believed he was contending only against the abuses and errors of the Mediæval Church. He declared, no doubt, not infrequently that he was not satisfied with the “dear Fathers,” and that they had all gone astray; yet he was not clear-sighted enough to say to himself that if the Church Fathers were in error, their decrees at the Councils could not possibly contain the whole truth. In no way, it is true, did he feel himself any longer externally bound by these decrees, nay, we can see brilliant flashes of incisive criticism, e.g. in his treatise on Councils and Churches; yet these continued on the whole without effect.
He always fell back again upon the view that the wretched Pope was alone to blame for all the evil, and that all the mischief, therefore, was connected with the Middle Ages only. Thus from this side his prepossession in favour of the faith-formulæ of the Ancient Church—on the ground that they did not take to do with works and law—was only further strengthened; indeed there was exercising its influence here, unconsciously to himself, a remnant of the idea that the empirical Church is authority. — Adolf von Harnack
It is precisely here that Luther is less Reformer than Zwingli or Calvin. Luther wanted to fix the Papacy, nothing more. Zwingli and Calvin wanted to return to the Christianity of the New Testament. Luther never would have gone that far had he lived to the present day.