3 And wherefore, my beloved? For it is right that we should search into the apostolic mind. Not only in the beginning of the Epistles, but towards their close, and in the middle of them, he used persuasions and admonitions. I hope therefore that, by your prayers, I shall in no respect falsely represent the plan of that holy man. As he was well skilled in these divine matters, and knew the power of the divine teaching, he deemed it necessary, in the first place, to make known the word concerning Christ, and the mystery regarding Him; and then afterwards to point to the correction of habits, so that when they had learned to know the Lord, they might earnestly desire to do those things which He commanded.
For when the Guide to the laws is unknown, one does not readily pass on to the observance of them. Faithful Moses, the minister of God, adopted this method; for when he promulgated the words of the divine dispensation of laws, he first proclaimed the matters relating to the knowledge of God: 'Hear, O Israel, the Lord your God is one Lord.' Afterwards, having shadowed Him forth to the people, and taught of Him in Whom they ought to believe, and informed their minds of Him Who is truly God, he proceeds to lay down the law relating to those things whereby a man may be well-pleasing to Him, saying, 'You shall not commit adultery; you shall not steal;' together with the other commandments.
For also, according to the Apostolic teaching, 'He that draws near to God must believe that He is, and that He is a rewarder of them that seek Him.' Now He is sought by means of virtuous deeds, as the prophet says; 'Seek the Lord, and when you have found Him, call upon Him; when He is near to you, let the wicked forsake his ways, and the lawless man his thoughts.'
Source: Letters (New Advent)