6 Why have I said this? For perhaps after these words one may justly say to me: Lay aside the book then. Why do you take in hand what exceeds your measure? Why trust your tongue to it? To this I reply: Many heretics abound; and God has permitted them to abound to this end, that we may not be always nourished with milk and remain in senseless infancy. For inasmuch as they have not understood how the divinity of Christ is set forth to our acceptance, they have concluded according to their will: and by not discerning aright, they have brought in most troublesome questions upon catholic believers; and the hearts of believers began to be disturbed and to waver.
Then immediately it became a necessity for spiritual men, who had not only read in the Gospel anything respecting the divinity of our Lord Jesus Christ, but had also understood it, to bring forth the armor of Christ against the armor of the devil, and with all their might to fight in most open conflict for the divinity of Christ against false and deceitful teachers; lest, while they were silent, others might perish. For whoever have thought either that our Lord Jesus Christ is of another substance than the Father is, or that there is only Christ, so that the same is Father, Son, and Holy Spirit; whoever also have chosen to think that He was only man, not God made man, or God in such wise as to be mutable in His Godhead, or God in such wise as not to be man; these have made shipwreck from the faith, and have been cast forth from the harbor of the Church, lest by their inquietude they might wreck the ships in their company.
Which thing obliged that even we, though least and as regards ourselves wholly unworthy, but in regard of His mercy set in some account among His stewards, should speak to you what either you may understand and rejoice with me, or, if you cannot yet understand, by believing it you may remain secure in the harbor.
Source: Tractates on the Gospel of John (New Advent)