1 Wherefore also Paul, since he was the apostle of the Gentiles, says, “I laboured more than they all.” For the instruction of the former, [viz., the Jews,] was an easy task, because they could allege proofs from the Scriptures, and because they, who were in the habit of hearing Moses and the prophets, did also readily receive the First-begotten of the dead, and the Prince of the life of God, — Him who, by the spreading forth of hands, did destroy Amalek, and vivify man from the wound of the serpent, by means of faith which was [exercised] towards Him. As I have pointed out in the preceding book, the apostle did, in the first place, instruct the Gentiles to depart from the superstition of idols, and to worship one God, the Creator of heaven and earth, and the Framer of the whole creation; and that His Son was His Word, by whom He founded all things; and that He, in the last times, was made a man among men; that He reformed the human race, but destroyed and conquered the enemy of man, and gave to His handiwork victory against the adversary. But although they who were of the circumcision still did not obey the words of God, for they were despisers, yet they were previously instructed not to commit adultery, nor fornication, nor theft, nor fraud; and that whatsoever things are done to our neighbours' prejudice, were evil, and detested by God. Wherefore also they did readily agree to abstain from these things, because they had been thus instructed.
2. But they were bound to teach the Gentiles also this very thing, that works of such a nature were wicked, prejudicial, and useless, and destructive to those who engaged in them. Wherefore he who had received the apostolate to the Gentiles, did labour more than those who preached the Son of God among them of the circumcision. For they were assisted by the Scriptures, which the Lord confirmed and fulfilled, in coming such as He had been announced; but here, [in the case of the Gentiles,] there was a certain foreign erudition, and a new doctrine [to be received, namely], that the gods of the nations not only were no gods at all, but even the idols of demons; and that there is one God, who is “above all principality, and dominion, and power, and every name which is named;” and that His Word, invisible by nature, was made palpable and visible among men, and did descend “to death, even the death of the cross;” also, that they who believe in Him shall be incorruptible and not subject to suffering, and shall receive the kingdom of heaven. These things, too, were preached to the Gentiles by word, without [the aid of] the Scriptures: wherefore, also, they who preached among the Gentiles underwent greater labour. But, on the other hand, the faith of the Gentiles is proved to be of a more noble description, since they followed the word of God without the instruction [derived] from the [sacred] writings (sine instructione literarum).
Source: Adversus haereses (New Advent)