The presiding ministers of the “mystery of godliness” have need of a system in their instructions, in order that the Church may be replenished by the accession of such as should be saved, through the teaching of the word of Faith being brought home to the hearing of unbelievers. Not that the same method of instruction will be suitable in the case of all who approach the word. The catechism must be adapted to the diversities of their religious worship; with an eye, indeed, to the one aim and end of the system, but not using the same method of preparation in each individual case.
The Judaizer has been preoccupied with one set of notions, one conversant with Hellenism, with others; while the Anomœan, and the Manichee, with the followers of Marcion, Valentinus, and Basilides, and the rest on the list of those who have wandered into heresy, each of them being prepossessed with their peculiar notions, necessitate a special controversy with their several. opinions. The method of recovery must be adapted to the form of the disease. You will not by the same means cure the polytheism of the Greek, and the unbelief of the Jew as to the Only-begotten God: nor as regards those who have wandered into heresy will you, by the same arguments in each case, upset their misleading romances as to the tenets of the Faith.
No one could set Sabellius right by the same instruction as would benefit the Anomœan. The controversy with the Manichee is profitless against the Jew. It is necessary, therefore, as I have said, to regard the opinions which the persons have taken up, and to frame your argument in accordance with the error into which each has fallen, by advancing in each discussion certain principles and reasonable propositions, that thus, through what is agreed upon on both sides, the truth may conclusively be brought to light.
When, then, a discussion is held with one of those who favour Greek ideas, it would be well to make the ascertaining of this the commencement of the reasoning, i.e. whether he presupposes the existence of a God, or concurs with the atheistic view. Should he say there is no God, then, from the consideration of the skilful and wise economy of the Universe he will be brought to acknowledge that there is a certain overmastering power manifested through these channels. If, on the other hand, he should have no doubt as to the existence of Deity, but should be inclined to entertain the presumption of a plurality of Gods, then we will adopt against him some such train of reasoning as this: “does he think Deity is perfect or defective?” and if, as is likely, he bears testimony to the perfection in the Divine nature, then we will demand of him to grant a perfection throughout in everything that is observable in that divinity, in order that Deity may not be regarded as a mixture of opposites, defect and perfection.
But whether as respects power, or the conception of goodness, or wisdom and imperishability and eternal existence, or any other notion besides suitable to the nature of Deity, that is found to lie close to the subject of our contemplation, in all he will agree that perfection is the idea to be entertained of the Divine nature, as being a just inference from these premises. If this, then, be granted us, it would not be difficult to bring round these scattered notions of a plurality of Gods to the acknowledgment of a unity of Deity.
For if he admits that perfection is in every respect to be ascribed to the subject before us, though there is a plurality of these perfect things which are marked with the same character, he must be required by a logical necessity, either to point out the particularity in each of these things which present no distinctive variation, but are found always with the same marks, or, if (he cannot do that, and) the mind can grasp nothing in them in the way of particular, to give up the idea of any distinction.
For if neither as regards “more and less” a person can detect a difference (in as much as the idea of perfection does not admit of it), nor as regards “worse” and “better” (for he cannot entertain a notion of Deity at all where the term “worse” is not got rid of), nor as regards “ancient” and “modern” (for what exists not for ever is foreign to the notion of Deity), but on the contrary the idea of Godhead is one and the same, no peculiarity being on any ground of reason to be discovered in any one point, it is an absolute necessity that the mistaken fancy of a plurality of Gods would be forced to the acknowledgment of a unity of Deity.
For if goodness, and justice, and wisdom, and power may be equally predicated of it, then also imperishability and eternal existence, and every orthodox idea would be in the same way admitted. As then all distinctive difference in any aspect whatever has been gradually removed, it necessarily follows that together with it a plurality of Gods has been removed from his belief, the general identity bringing round conviction to the Unity.
Source: The Great Catechism (New Advent)