Christian Trinitarian arguments were strong defensively but useless offensively.

Evidence

His constant insistence upon Trinitarian argument indicates the very narrow limits of debate within which he was prepared to argue, and is one of the best examples of Christian unwillingness to venture away from familiar ground. Christian arguments were sound enough defensively thus they easily recognised Muhammad’s objection that the Sonship of God is anthropomorphic, and refuted the objection as anthropomorphic itself. Offensively these ‘proofs’ were useless. — Norman Daniel, Islam and the West, 1430