1 This title does not require a fresh consideration: for the meaning of, “to the end,” has already been sufficiently handled. Let us then look to the text itself of the Psalm, which to me appears to be sung against the heretics, who, by rehearsing and exaggerating the sins of many in the Church, as if either all or the majority among themselves were righteous, strive to turn and snatch us away from the breasts of the one True Mother Church: affirming that Christ is with them, and warning us as if with piety and earnestness, that by passing over to them we may go over to Christ, whom they falsely declare they have. Now it is known that in prophecy Christ, among the many names in which notice of Him is conveyed in allegory, is also called a mountain. We must accordingly answer these people, and say, “I trust in the Lord: how say ye to my soul, Remove into the mountains as a sparrow?”. I keep to one mountain wherein I trust, how say ye that I should pass over to you, as if there were many Christs? Or if through pride you say that you are mountains, I had indeed need to be a sparrow winged with the powers and commandments of God: but these very things hinder my flying to these mountains, and placing my trust in proud men. I have a house where I may rest, in that I trust in the Lord. For even “the sparrow has found her a house,” and, “The Lord has become a refuge to the poor.” Let us say then with all confidence, lest while we seek Christ among heretics we lose Him, “In the Lord I trust: how say ye to my soul, Remove into the mountains as a sparrow?”
2. “For, lo, sinners have bent the bow, they have prepared their arrows in the quiver, that they may in the obscure moon shoot at the upright in heart”. These be the terrors of those who threaten us as touching sinners, that we may pass over to them as the righteous. “Lo,” they say, “the sinners have bent the bow:” the Scriptures, I suppose, by carnal interpretation of which they emit envenomed sentences from them. “They have prepared their arrows in the quiver:” the same words, that is, which they will shoot out on the authority of Scripture, they have prepared in the secret place of the heart. “That they may in the obscure moon shoot at the upright in heart:” that when they see, from the Church's light being obscured by the multitude of the unlearned and the carnal, that they cannot be convicted, they may corrupt good manners by evil communications. But against all these terrors we must say, “In the Lord I trust.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)