Medieval thinkers treated any non-Trinitarian concept of God as wholly defective

Evidence

A falsely Augustinian cast of thought led some authors to ‘prove’ the Trinity by reason, and so to try to convict Islam of being irrational. No mediaeval author could see any concept of God that was not Trinitarian as other than wholly defective, and there was only a narrow margin between rational explanations and rational ‘proofs’ of the Trinity. — Norman Daniel, Islam and the West, 974