Tadeusz Klimski studied three ancient and medieval authors: Saint Augustine, Johannes Scotus Eriugena and Honorius of Autun, and devoted them his publications in „Studia Philosophiae Christianae”. He studied in their texts the issues of truth, proceeded from Aristotle who defined philosophy as „the knowledge of the truth”. According to T. Klimski, knowability of truth was by St. Augustine radically dependent on God, and in the philosophy of Eriugena truth was equated directly with him. Honorius of Autun in his works connected the truth with the essence of things, however, he has not seen existential aspect resulting from the definition of God as semper esse. According to T. Klimski, these philosophers, because they were neoplatonists they haven’t come to the truth with existence, with esse, as something most fundamental for the understanding of being.