4 Next we must enquire, what in this Psalm is meant by “the obscure moon,” in which sinners have prepared to shoot at the upright in heart? For not in one way only may the moon be said to be obscure: for when her monthly course is finished, and when her brightness is interrupted by a cloud, and when she is eclipsed at the full, the moon may be called obscure. It may then be understood first of the persecutors of the Martyrs, for that they wished in the obscure moon to shoot at the upright in heart; whether it be yet in the time of the Church's youth, because she had not yet shone forth in greatness on the earth, and conquered the darkness of heathen superstitions; or by the tongues of blasphemers and such as defame the Christian name, when the earth was as it were beclouded, the moon, that is, the Church, could not be clearly seen; or when by the slaughter of the Martyrs themselves and so great effusion of blood, as by that eclipse and obscuration, wherein the moon seems to exhibit a bloody face, the weak were deterred from the Christian name; in which terror sinners shot out words crafty and sacrilegious to pervert even the upright in heart. And secondly, it can be understood of these sinners, whom the Church contains, because at that time, taking the opportunity of this moon's obscurity, they committed many crimes, which are now tauntingly objected to us by the heretics, whereas their founders are said to have been guilty of them. But howsoever that be which was done in the obscure moon, now that the Catholic name is spread and celebrated throughout the whole world, what concern of mine is it to be disturbed by things unknown? For “in the Lord I trust;” nor do I listen to them that say to my soul, “Remove into the mountains as a sparrow. For, lo, sinners have bent the bow, that they may in the obscure moon shoot at the upright in heart.” Or if the moon seem even now obscure to them, because they would make it uncertain which is the Catholic Church, and they strive to convict her by the sins of those many carnal men whom she contains; what concern is this to him, who says in truth, “In the Lord I trust”? By which word every one shows that he is himself wheat, and endures the chaff with patience unto the time of winnowing.
5. “In the Lord,” therefore, “I trust.” Let them fear who trust in man, and cannot deny that they are of man's party, by whose grey hairs they swear; and when in conversation it is demanded of them, of what communion they are, unless they say that they are of his party, they cannot be recognised....Or perhaps you will say that it is written, “You shall know them by their works”? I see indeed marvellous works the daily violences of the Circumcelliones, with the bishops and presbyters for their leaders, flying about in every direction, and calling their terrible clubs “Israels;” which men now living daily see and feel. But for the times of Macarius, respecting which they raise an invidious cry, most men have not seen them, and no one sees them now: and any Catholic who saw them could say, if he wished to be a servant of God, “In the Lord I trust.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)