Making a commentary, line-by-line, on the Bible, searching for clarity on literally every single word. But in addition to writing between the lines, writing in the margins was often necessary, and this commentary became formalized as a gloss with a Biblical passage in the center of a page surrounded by commentary filling every single margin.
Lombard's textbook The Sentences became the fundamental textbook of the entire West. Peter summarized it, The four books of the Sentences first deals with the Trinity, the notion of a god that is both one and three. Second he deals with the creation, fall and redemption by Christ of man. He has dealt with the creator, then the creation. Then the third and fourth books deal with redemption, grace and the sacraments.
The order of reason demands that we who, in the first book, have said something about the inexplicable mystery of the most high Trinity, through the irrefragable witness of hte Saints, and then, in the second book, have thoroughly penetrated into the order of the creation of things and the fall of man, following the model of certain authorities, consequently in the third and fourth books examine his redemption, accomplished through the grace of the Mediator of God and men, as well as the sacraments of human redemption by which the bruises of man are bound up and the wounds of sinners are healed: just as the Samaritan comes to the wounded, the physician to the sick and grace to the wretched.
Thus, Lombard gives us Christian theology in 933 chapters in four books. It has clear layout and structure with clear chapter headings, a novel approach for Peter Lombard's time. Most in Lombard's time lacked even title pages stating the author; he was a great force in putting together the organization structure needed to make sense of a large piece of writing. It is not anti-philosophical, but it is not philosophically deep: it is the work of a professional theologian. When Lombard speaks of theology, he is more interested in Biblical content than Biblical linguistics. If he has learned from Augustine, Jerome, Hillary or Gregory, he mnetions them; but he does not mention his contemporaries. He addresses and tries to resolve conflicts.
Lombard is intersted in the Trinity, the mysterious core of Christianity. Much was written in the Bible about the Father and the Son, but much less about the Holy Spirit; thus, the latter was most open to speculation. God had many names. God's relationship to humans through god's knowledge and will. Lombard speaks of god's foreknowledge, providential knowledge, predestined knowledge; god's eminence, being in the universe; god's being everywhere, his omnipresence in the universe; god's creation of the universe, and thus transcendent existence outside of the universe. How could one reconcile god being present everywhere, but also on a higher plane? How does an all-knowing, all-present god create such a messy universe? Facing these divine mysteries, Lombard concludes that the human mind is weak but that god reveals itself in the Bible.
Lombard returns to Chapter 1 Verse 20 from Epistles of ROMans, which meant so much to Augustine about god invisible. Peter mentions four ways of knowing the creator from created things:
since some parts of creatino, heaven and earth, cna't have been made by any creature, a creator must have made them.
created things, but changeable things come to be only be from what exist without changing.
All substances are either bodies or spirits. Spirit is body, but better than both is what creates both.
All forms of things are either sensible or intelligible, and spirit is more like form than body is like form, but all things get their form from a first and unchangeable form.
Peter ended up right in the middle of theological and philosophical divides and was extraordinarily influential.