Evolution of Doctrine: Simultaneously righteous and sinner
An important conceptual change in theological doctrine was Luther's idea of "Simultaneously righteous and sinner" (Simul Iustus et Peccator).
In early Christianity, human righteousness is conceived of as being on a single continuum, with extreme wickedness at one end and godly righteousness at the other end. It is thus a grey-scale which measures morality:
Sinner <------------------------------------> Righteous
The basic idea is that a given human can only occupy one location on this scale at a particular time. Over time they can become better or worse - moral improvement, or moral decline. God is seen as approving of righteousness and disapproving of wickedness. The solution to avoiding God's anger is thus to move across the scale and become a better person, ie to repent of one's wicked ways and change them. This continuum is taken for granted by Christians throughout the first millennia: To become righteous is to cease being a sinner, and vice versa. There is never the thought of a "righteousness" that doesn't entail actually being moral and ceasing from sin.
Luther however took an axe to this continuum and cut it into two. In his system there are two such continua: One measuring human morality as it actually is, and one measuring God's (judicial) view of humans. On one continuum we can be sitting at "world's worst sinner" and on the other at "perfectly righteous" - our true moral state, and our moral state before God, are in Luther's system two totally different things.
The practical outworking of this is that there is no great need for humans to be actually righteous or live righteously, and thus Luther writes "boldly sin... No sin will separate us from the Lamb, even though we commit fornication and murder a thousand times a day." This is a strikingly different attitude toward sinning compared to that evident in the first millennia writings (see the Desert Fathers for example).
The essential conceptual development at work here is the breaking of the old single-continuum into two, so that you can now be both sinner and saint at the same time, where in previous Christianity being one excluded being the other. In a sense it was Luther's projection of this new model back onto Paul's writings that shaped his entire theology. Paul was reread in light of this double-continua and what now came out of his writings was no longer talk of actual moral righteousness, but rather a way to be righteous before God despite actually being a sinner. Paul's gospel, in Luther's model, is then about how human beings can be righteous before God and sinners in actuality at the same time due to what Christ has achieved. Actual moral change is no longer a prerequisite to salvation, because the continuum that governs salvation and status in the eyes of God is now an independent continuum to that which measures our actual morality.