The Self-Donation of God, pg. 3-4:
"This means that we will expound the Scriptures according to the sensus literalis, that is, the literal sense. This is by no means identical with the "literalism" or perhaps even better, “letterism” as the Wittenberg Reformers were well aware. In his definition of the literal sense, Thomas Aquinas claimed that the literal sense was the meaning which God intended when he communicated the content of the Bible through the inspired authors. Doubtless, the Reformers would not have disagreed with such a sentiment.
To show how the Reformers understood this intended meaning, we should turn to Luther's concept of scriptural clarity. Luther spoke about two kinds of clarity, external clarity and inner clarity. The external clarity, claimed Luther, was the grammatical and hence historically accessible meaning of the text. Such a meaning was open to anyone. The inner clarity was the meaning of the Bible as it centered on Christ. Since one cannot understand Christ or see the unity of the Bible in him without the Holy Spirit (2 Cor 3) those who read the Scripture without faith fail to grasp its true meaning. Conversely, it is also true that one will not understand the Scriptures if one does not understand their mode of speaking and grammar, which are of course, historically conditioned.
We can therefore see what the sensus literalis is for Luther in light of Christ. On the one hand God communicated himself in the concrete, contextual meaning of the text for the people to whom he addressed it through the prophets and apostles. At the same time, he intended that that meaning might also bear witness to Christ and to ultimately drive people to him. Therefore the literal sense is the coming together of the external and internal clarity of the Bible, just as when we refer to the person of Christ in the concrete we speak about the unity of his two natures. The literal sense is not, as modern interpreters have often thought, the meaning of the text as we might want to construe it based on the limited circumstances of certain historical authors. Rather, it is the harmony of the literal, grammatical meaning of the words of the Bible, together with the larger narrative of the history of salvation, culminating in and centering in Jesus Christ. This conception of the Bible is consummate with the Lutheran doctrine of the genus majestaticum, wherein the divine nature (in analogy to the internal clarity) is not something separate from the human nature (in analogy to the external clarity), but rather communicates the fullness of itself through external form of the human nature."