Easter vigil. That could be a reason, as was suggested there (see Sermon 19, note 1), for dating it to
SERMON 223B 221 in which he slept for our sakes. He, as a matter of fact, announced his passion a long, long time before through the prophet, when he said, I myself slept, and I rose up, because the Lord upheld me (Ps 3:5). He called the Father Lord. Thus we keep awake on the night in which he slept, in order to live by the death which he suffered; we celebrate a vigil over his temporal slumber, so that as he watches for our sake, we may rise again to an eternal vigil and remain tirelessly awake forever.
However, he also rose again on this night, and it is in expectation of that resurrection that we are keeping watch. He was handed over for our sins, and he slept. He rose again for our justification (Rom 4:25). That's why we, on this one night in which he slept, all throng to this vigil of ours, so that safe in his watchful keeping we may some time or other be quit of that sort of sleep; and by keeping awake we await the hour of his awakening, in case at the very point of our justification, for which he rose again, carelessness should creep in, and our hearts, not our bodies, should fall asleep. So let us watch, my dearest friends, and pray, lest we enter into temptation.
NOTES
1. Preached, presumably, like Sermon 219 and others we have seen, at the beginning of the Easter vigil. That could be a reason, as was suggested there (see Sermon 19, note 1), for dating it to one of the years before he was ordained bishop, that is to 391-396. But for qualification of this admitttedly tenuous inference, see Sermon 222, note 1.
2. Another such reason, I would like to think, is this whiff of anti-Judaism, which I want to believe Augustine to some extent grew out of in his later years. See Sermon 218B, note 9; and in slight extenuation Sermon 218, note 13.