Start with the article on Justification (articulus stantis et cadentis ecclesiae) and work your way outward. Read, mark, learn, and inwardly digest Article IV in the Augsburg Confession and the Apology of the Augsburg Confession. Get Justification right, and correcting the abuses in the Mass, Penance, etc., will soon follow.
OK, good.
But don't leave me hanging. To use your words, that's a "start". Where might this lead?
Let me ask this question another way.
Here is an article I posted a few days ago. It's called SBC Leader Sees Calvinism As Top Challenge". Its general gist is that the CEO of the SBC Executive Committee sees Calvinism as a danger to the Baptist endeavor and sees their teachings as "the work of men".
Let's say my examination of the Augsburg Confession led me into the Southern Baptist fold. Would I have now found "pure doctrine" (your words)? Or would that be found in Calvinism? Maybe both? Maybe neither? Perhaps Lutheranism would be the answer?
This is precisely the point of the article.
Love ya brother, (especally DUFU) but here is the flaw in Lutheran and reformed covenant theology. If we claim sola scriptura and then go to some historic confession or creed we are no longer sola scriptura. For reformed to refer to the WCF (for example) puts them in the same boat so to speak as the catholics when they go to the traditions of the church. I know there are well reasoned rebuttles to this statement, but on this topic, Here I Stand.