3 Nothing is more harassing to human nature than the sense of impending danger. If calamities unknown or unanticipated befall us, we may need pity, yet we have been free from care; no load of anxiety has oppressed us. But he whose mind is full of possibilities of trouble suffers already a torment in his fear. I who now am venturing out to sea, am a mariner not unused to shipwreck, a traveller who knows by experience holy brigands lurk in the forests, an explorer of African deserts aware of the danger from scorpions and asps and basilisks. I enjoy no instant of relief from the knowledge and fear of present danger.
Every heretic is on the watch, noting every word as it drops from my mouth. The whole progress of my argument is infested with ambuscades and pitfalls and snares. It is not of the road, of its hardness or steepness, that I complain; I am following in the footsteps of the Apostles, not choosing my own path. My trouble is the constant peril, the constant dread, of wandering into some ambush, of stumbling into some pit, of being entangled in some net. My purpose is to proclaim the unity of God, in the sense of the Law and Prophets and Apostles.
Sabellius is at hand, eager with cruel kindness to welcome me, on the strength of this unity, and swallow me up in his own destruction. If I withstand him, and deny that, in the Sabellian sense, God is One a fresh heresy is ready to receive me, pointing out that I teach the existence of two Gods. Again, if I undertake to tell how the Son of God was born from Mary, Photinus, the Ebion of our day, will be prompt to twist this assertion of the truth into a confirmation of his lie.
I need mention no other heresies save one; all the world knows that they are alien from the Church. It is one that has been often denounced, often rejected, yet it preys upon our vitals still. Galatia has reared a large brood of godless assertors of the unity of God. Alexandria has sown broadcast, over almost the whole world, her denial, which is an affirmation, of the doctrine of two Gods. Pannonia upholds her pestilent doctrine that the only birth of Jesus Christ was from the Virgin.
And the Church, distracted by these rival faiths, is in danger of being led by means of truth into a rejection of truth. Doctrines are being forced upon her for godless ends, which, according to the use that is made of them, will either support or overthrow the faith. For instance, we cannot, as true believers, assert that God is One, if we mean by it that He is alone; for faith in a lonely God denies the Godhead of the Son. If, on the other hand, we assert, as we truly can, that the Son is God, we are in danger, so they fondly imagine, of deserting the truth that God is One.
We are in peril on either hand; we may deny the unity or we may maintain the isolation. But it is a danger which has no terrors for the foolish things of the world. Our adversaries are blind to the fact that His assertion that He is not alone is consistent with unity; that though He is One He is not solitary.
Source: On the Trinity (New Advent)