Medieval apologists claimed pure reason could prove Christian doctrine

Evidence

Moreover, God could not have created either wisdom or will, since he needs both wisdom and will in order to create. Petrus equates substance with the Father, wisdom with the Son, and will with the Holy Spirit; the rationalistic explanation of the Creation, for Alfonsi, requires the existence of the Trinity. Tathlîth al-wahdânîyah gives essentially the same argument, though its triad differs slightly: it is power, knowledge, and will. Both authors are working within a well-established tradition of Arab Christian apologetics; both emphasize supposedly rational and scientific proofs. Reason (ratio) can disprove Judaism and Islam and prove the essence of Christian truth. — John V. Tolan, Saracens, 829

Categorization

• XREF: Parallels rational theology debates elsewhere, including Islamic kalam rationalism it draws from.
• SEED: An essay angle: the strange alliance of rationalist proof with theological dogma in medieval interfaith polemic.