The Development of the doctrine in the medieval period

For the current study, it is used to demarcate the period initiated by through Alaric’s conquest of Rome in 410, and the shift of the intellectual world to northern Europe. The shift in the intellectual center also witnesses a change in theological method, i.e. the assimilation of pagan philosophy and patristic theology. This period is characterized by the accumulation of biblical and patristic material considered relevant for particular theological issues and the attempt to resolve apparent contradictions encountered in the process.
The discussion of justification developed as the most appropriate metaphor for the articulation of soteriological convictions. There are two significant developments associated with this:
1. The transference of the discussion of salvation from mythological to moral or legal plane; and
2. The incorporation of Pauline concepts into the later medieval theological discussions.
The mythological discussion of salvation was criticized because it was incongruent with the Righteousness of God, hence, the shift to moral and legal plane. God, being righteous, must act in accordance to his righteousness in the redemption of man (legal/moral propriety) and the method must also be righteous.
The use of Pauline commentaries in the early medieval period influenced the later medieval period. These brought about two pertinent theological questions: salvation of OT patriarch and the relation between faith and work, which theologians have to discuss in relation to the concept of justification. The use of Pauline commentaries contributed to the development of the concept of justification as the most important soteriological concept because it was used by Paul in connection with those soteriological issues. The systemization of theology is the second factor that enhanced the importance of justification as the metaphor for soteriological discussions.

McGrath, Iustitia Dei, 37-40