“A Christian intellectual development occurs when all the truths of the Faith are impressed so deeply into the intellect that the intellect’s whole existence is made up of these truths alone. When it begins to reason over something, it reasons according to what it knows of the Christian truths, and would never make the slightest move without them. The Apostle calls this keeping the image of a sound mind (II Tim. 1:7).”

(St Theophan the Recluse ‘Path to Salvation’)