Origen studied under Clement in Alexandria. He wrote numerous books, but history primarily remembers him for his Hexapla, Against Celsus, and De Principiis.
His apology against Celsus works through the lens of fulfilled Old Testament prophecy, the incarnation not changing the nature of the pre-existent logos, and the rapid expansion of Christianity because of divine assistance.
His approach to exegesis feels free to go beyond the written words of Scripture, which brought out ideas like a spiritual and material creation, pre-existent souls, Satan being saved at the end of history, and a repeating cycle of creation, fall, and redemption. Though his ideas occasionally seem more Platonist than Christian, he does reject Marcion and the gnostics clearly in his works.