4 Wherefore it should be our ambition that the Word of Christ dwell in us richly. For it is not for one kind of battle only that we have to be prepared. This warfare is manifold, and is engaged with a great variety of enemies; neither do all these use the same weapons, nor do they practice the same method of attack; and he who has to join battle with all, must needs know the artifices of all, and be at once both archer and slinger, captain and general, in the ranks and in command, on foot and on horseback, in sea-fight and in siege.
In common warfare, indeed, each man repels the enemy by discharging the particular duty which he has undertaken. But here it is otherwise; and if any one wishes to come off conqueror in this warfare, he must understand all forms of the art, as the devil knows well how to introduce his own assailants through any one spot which may happen to be unguarded, and to carry off the sheep. But not so where he perceives the shepherd coming equipped with accurate knowledge at all points, and well acquainted with his plottings.
Wherefore we ought to be well-guarded in all parts: for a city, so long as it happens to be surrounded with a wall, laughs to scorn the besiegers, abiding in great security; but if any one makes a breach in the wall, though but of the size of a gate, the rest of the circuit is of no use, although the whole of it stand quite securely; so it is with the city of God: so long as the presence of mind and wisdom of the shepherd, which answers to the wall, protect it on all sides, all the enemy's devices end in his confusion and ridicule, and they who dwell within the wall abide unmolested, but wherever any one has been able to demolish a single part, though the rest stand never so fast, through that breach ruin will enter upon the whole.
For to what purpose does a man contend earnestly with the Greeks, if at the same time he becomes a prey to the Jews? Or get the better of both these and then fall into the clutches of the Manichæans? or after he has proved himself superior to them even, if they who introduce fatalism enter in, and make havoc of the flock? But not to enumerate all the heresies of the devil, it will be enough to say that unless the shepherd is well skilled in refuting them all, the wolf, by means of any one of them, can enter, and devour the greater part of the flock.
In ordinary warfare we must always look for victory being won or defeat sustained by the soldiers who are on the field of battle. But in the spiritual warfare the case is quite different. For there it often happens that the combat with one set of enemies secures a victory for others who never engaged in battle at all, nor took any trouble, but were sitting still all the while; and he who has not much experience in such occurrences will get pierced, so to say, with his own sword, and become the laughing-stock of friends and foes alike.
I will try by an example to make clear what I am saying. They who receive the wild doctrines of Valentinus and Marcion, and of all whose minds are similarly diseased, exclude the Law given by God to Moses from the catalogue of the Divine Scriptures. But Jews so revere the Law, that although the time has come which annuls it, they still contend for the observance of all its contents, contrary to the purpose of God. But the Church of God, avoiding either extreme, has trodden a middle path, and is neither induced on the one hand to place herself under its yoke, nor on the other does she tolerate its being slandered, but commends it, though its day is over, because of its profitableness while its season lasted.
Now it is necessary for him who is going to fight with both these enemies, to be fully conversant with this middle course. For if in wishing to teach the Jews that they are out of date in clinging to the old law, he begins to find fault with it unsparingly, he gives no little handle to those heretics who wish to pull it to pieces; and if in his ambition to stop their mouths he extols it immoderately, and speaks of it with admiration, as necessary for this present time, he unseals the lips of the Jews.
Again they who labor under the frenzy of Sabellius and the craze of Arius, have both fallen from a sound faith for want of observing a middle course. The name of Christian is applied to both these heretics; but if any one examines their doctrines, he will find the one sect not much better than the Jews, and differing from them only in name, and the other very nearly holding the heresy of Paul of Samosata, and that both are very wide of the truth. Great, therefore, is the danger in such cases, and the way of orthodoxy is narrow and hemmed in by threatening crags on either side, and there is no little fear lest when intending to strike at one enemy we should be wounded by the other.
For if any one assert the unity of the Godhead, Sabellius straightway turns that expression to the advantage of his own mental vagary, and if he distinguish the Persons, and say that the Father is one, and the Son another, and the Holy Spirit a third, up gets Arius, ready to wrest that distinction of Persons into a difference of substance; so we must turn and flee both from the impious confounding of the Persons by the one, and the senseless division of the substance by the other, confessing, indeed, that the Godhead of the Father and of the Son and of the Holy Ghost, is all one, while we add thereunto a Trinity of Persons.
For then we shall be able to fortify ourselves against the attacks of both heretics. I might tell you besides these, of several other adversaries against which, except we contend bravely and carefully, we shall leave the field covered with wounds.
Source: On the Priesthood (New Advent)