But how, beloved, will this be done? It will be done only by the following things: if our understanding is fixed by faith towards God, if we earnestly seek the things that are pleasing and acceptable to Him, if we do the things that are in harmony with His blameless will, and if we follow the way of truth, casting away from us all unrighteousness and iniquity. Second Clement is the oldest sermon we have outside of the New Testament. We don't even know who gave it.
It was preserved along with the letter from Clement of Rome, and so people attributed it to Clement. It could have been from him. It's certainly no older than, say, about the year 140 AD. It says, "For he is faithful who has promised that he will reward everyone according to his works. If therefore we shall do righteousness in the sight of God, we shall enter into his kingdom."