1 The Psalm which we have sung is short: but as it is written in the Gospel of Zacchæus that he was “little of stature,” but mighty in works; as it is written of that widow who cast two mites into the treasury, little was the money, but great was her charity; thus also this Psalm, if you count the words, is short; if you weigh the sentiments, is great....Let the Spirit of God speak, let It speak to us, let It sing to us; whether we wish or wish not to dance, let It sing. For as he who dances, moves his limbs to the time; so they who dance according to the commandment of God, in their works obey the sound. What therefore says the Lord in the Gospel to those who refuse to do this? “We have piped unto you, and you have not danced: we have mourned unto you, and you have not lamented.” Let Him therefore sing; we trust in God's mercy, for there will be those by whom He consoles us. For they who are obstinate, continuing in wickedness, although they hear the Word of God, by their offenses daily disturb the Church. Of such this Psalm speaks; for thus it begins.
2. “Many a time have they fought against me from my youth up”. The Church speaks of those whom She endures: and as if it were asked, “Is it now?” The Church is of ancient birth: since saints have been so called, the Church has been on earth. At one time the Church was in Abel only, and he was fought against by his wicked and lost brother Cain. At one time the Church was in Enoch alone: and he was translated from the unrighteous. At one time the Church was in the house of Noah alone, and endured all who perished by the flood, and the ark alone swam upon the waves, and escaped to shore. At one time the Church was in Abraham alone, and we know what he endured from the wicked. The Church was in his brother's son, Lot, alone, and in his house, in Sodom, and he endured the iniquities and perversities of Sodom, until God freed him from amidst them. The Church also began to exist in the people of Israel: She endured Pharaoh and the Egyptians. The number of the saints began to be also in the Church, that is, in the people of Israel; Moses and the rest of the saints endured the wicked Jews, the people of Israel. We come unto our Lord Jesus Christ: the Gospel was preached in the Psalms....For this reason, lest the Church wonder now, or lest any one wonder in the Church, who wishes to be a good member of the Church, let him hear the Church herself his Mother saying to him, Marvel not at these things, my son: “Many a time have they fought against me from my youth up.”
3. “Now may Israel say.” She now seems to be speaking of herself: for she seemed not to have commenced herself, but to have answered. But to whom has she replied? To them that think and say, How great evils do we endure, how great are the scandals that every day thicken, as the wicked enter into the Church, and we have to endure them? But let the Church reply through some, that is, through the voice of the stronger, let her reply to the complaints of the weak, and let the stable confirm the unstable, and the full-grown the infant, and let the Church say, “Many a time have they vexed me from my youth up”. Let the Church say this: let her not fear it. For what is the meaning of this addition, “From my youth up,” after the words, “Many a time have they fought against me”? At present the old age of the Church is assailed: but let her not fear. Hath she then failed to arrive at old age, because they have not ceased to fight against her from her youth up? Have they been able to blot her out? Let Israel comfort herself, let the Church console herself with past examples. Why have they fought against me? “For they could not prevail against me.”
4. “Upon my back have sinners built; they have done their iniquity afar off”. Why have they fought against me? Because “they could not prevail upon me.” What is this? They could not build upon me. I consented not with them unto sin. For every wicked man persecutes the good on this account, because the good man consents not with him to evil. Suppose he do some evil, and the Bishop censure him not, the Bishop is a good man: suppose the Bishop censure him, the Bishop is a bad man. Suppose he carry off anything, let the man robbed be silent, he is a good man: let him only speak and rebuke, even though he does not reclaim his goods, he is everything bad. He is bad then who blames the robber, and he is good who robs!...Heed not that such an one speaks to you: it is a wicked man through whom It speaks to you; but the word of God, that speaks to you, is not wicked. Accuse God: accuse Him, if you can.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)