That the one God was foretold even by the prophets
The prophets, who were very many, proclaim and declare the one God; for, being filled with the inspiration of the one God, they predicted things to come, with agreeing and harmonious voice. But those who are ignorant of the truth do not think that these prophets are to be believed; for they say that those voices are not divine, but human. Forsooth, because they proclaim one God, they were either madmen or deceivers. But truly we see that their predictions have been fulfilled, and are in course of fulfilment daily; and their foresight, agreeing as it does to one opinion, teaches that they were not under the impulse of madness. For who possessed of a frenzied mind would be able, I do not say to predict the future, but even to speak coherently? Were they, therefore, who spoke such things deceitful? What was so utterly foreign to their nature as a system of deceit, when they themselves restrained others from all fraud? For to this end were they sent by God, that they should both be heralds of His majesty, and correctors of the wickedness of man.
Moreover, the inclination to feign and speak falsely belongs to those who covet riches, and eagerly desire gains—a disposition which was far removed from those holy men. For they so discharged the office entrusted to them, that, disregarding all things necessary for the maintenance of life, they were so far from laying up store for the future, that they did not even labour for the day, content with the unstored food which God had supplied; and these not only had no gains, but even endured torments and death. For the precepts of righteousness are distasteful to the wicked, and to those who lead an unholy life. Wherefore they, whose sins were brought to light and forbidden, most cruelly tortured and slew them. They, therefore, who had no desire for gain, had neither the inclination nor the motive for deceit. Why should I say that some of them were princes, or even kings, upon whom the suspicion of covetousness and fraud could not possibly fall, and yet they proclaimed the one God with the same prophetic foresight as the others?
Source: The Divine Institutes (New Advent)