1 For, behold, the Jews are enemies, whom this Psalm seems to imply; the law of God they hold, and therefore of them has been said, “Slay not them, lest sometime they forget Your law:” in order that the nation of Jews might remain, and by it remaining the number of Christians might increase. Throughout all nations they remain certainly, and Jews they are, nor have they ceased to be what they were: that is, this nation has not so yielded to Roman institutions, as to have lost the form of Jews; but has been subjected to the Romans so as that it still retains its own laws; which are the laws of God.
But what in their case has been done? “You tithe mint and cummin, and have forsaken the weightier matters of the law, mercy, and judgment, straining a gnat, but swallowing a camel.” This to them the Lord says. And in truth so they are; they hold the law, hold the Prophets; read all things, sing all things: the light of the Prophets therein they see not, which is Christ Jesus. Not only Him now they see not, when he is sitting in Heaven: but not even at that time saw they Him, when among them humble He was walking, and they were made guilty by shedding the blood of the Same; but not all.
This even today we commend to the notice of your Love. Not all: because many of them were turned to Him whom they slew, and by believing on Him, they obtained pardon even for the shedding of His blood: and they have given an example for men; how they ought not to despair that sin of whatsoever kind would be remitted to them, since even the killing of Christ was remitted to them confessing....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)