14 “My mouth shall tell out Your righteousness”: not mine. From thence I will add to all Your praise: because even that I am righteous, if righteous I am, is Your righteousness in me, not my own: for You justify the ungodly. “All the day long Your salvation.” What is, “Your salvation”? Let no one assume to himself, that he saves himself, “Of the Lord is Salvation.” Not any one by himself saves himself, “Vain is man's salvation.” “All the day long Your Salvation:” at all times.
Something of adversity comes, preach the Salvation of the Lord: something of prosperity comes, preach the Salvation of the Lord. Do not preach in prosperity, and hold your peace in adversity: otherwise there will not be that which has been said, “all the day long.” For all the day long is day together with its own night. Do we when we say, for example, thirty days have gone by, mention the nights also; do we not under the very term days include the nights also? In Genesis what was said?
“The evening was made, and the morning was made, one day.” Therefore a whole day is the day together with its own night: for the night does serve the day, not the day the night. Whatever you do in mortal flesh, ought to serve righteousness: whatever you do by the commandment of God, be it not done for the sake of the advantage of the flesh, lest day serve night. Therefore all the day long speak of the praise of God, to wit, in prosperity and in adversity; in prosperity, as though in the day time; in adversity, as though in the night time: all the day long nevertheless speak of the praise of God, so that you may not have sung to no purpose, “I will bless God at every time, always the praise of Him is in my mouth.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)