Complex sentence: As regards theology, his works gave Christianity an impulse the effect of which was felt for centuries; the doctrine of the Trinity supplied him with matter for the most finished exposition to be found among the works of the Doctors of the Church.
Split into simpler sentences: As regards theology, his works gave Christianity an impulse that was felt for centuries. The doctrine of the Trinity supplied him with matter for the most finished exposition to be found among the works of the Doctors of the Church.