A passage in one of his homilies on Ezekiel provides a clear statement of the way the Bible is to be understood:
1.9.30 It goes on, referring to this book: Which was written on the inside and the outside [Ezek. 2:10]. For the book of the Bible is written on the inside through allegory and the outside through history; on the inside through a spiritual understanding, on the outside through a mere literal sense suited to those who are still weak; on the inside, because it promises things which cannot be seen, on the outside because it lays down visible things through its upright precepts; on the inside because it promises heavenly things, on the outside because it orders in what way earthly things are worthy of contempt, whether we put them to use or flee from desiring them. For it says some things about heavenly secrets, and ordains other things concerning outward actions. And those things which it orders on the outside are quite clear, but those things which it tells of interior things cannot be understood in full. And so it is written: You stretch out heaven like a skin, you who cover its upper parts in the waters [Ps. 104:2f.]. For what is indicated by the word heaven but holy scripture? From it shine forth the sun of wisdom and the moon of knowledge, and the stars of examples and virtues from the ancient fathers. It is stretched out like a skin because, having been formed by its writers through the tongue of the flesh, it is explained before our eyes in words through the exposition of learned people. But what are indicated by the word waters but the most holy choirs of angels? It is written of them: And the waters which are above the heaven, let them praise the name of the Lord [Ps. 148:4f.]. And so it is that the Lord covers the upper parts of heaven in the waters, because the high things of the Bible, that is what it tells of the nature of God or eternal joys, are known only to the angels in private, while we are still ignorant of them. Therefore heaven is also stretched out before us, and nevertheless its upper parts are concealed, because some things in the Bible are already clear to us through the disclosure of the Spirit, while other things which can only be made known to the angels are still kept hidden from us. Nevertheless we already discern a part of these hidden things through a spiritual understanding, we have already received the pledge of the Holy Spirit, because while we do not know these things in their fullness, nevertheless we love with our deepest being, and we are fed with the nourishment of truth through the many spiritual understandings which are already known to us.