But we are sure that the judgment of God is according to truth against them who commit such things.
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This means that we are not unaware that God will judge these people in truth, for we judge them ourselves. If what they do is displeasing to us, how much more will it be so to God, who is truly just and efficient in carrying out his work…. Paul is instilling fear, so that although the ungodly say that God does not care, in fact he will judge the wicked and most severely render to each one according to his deserts, not sparing any. Commentary on Paul’s Epistles.
The truth, therefore, will be His, whose is also the wrath, which has to be revealed to avenge the truth. Likewise, when adding, "We are sure that the judgment of God is according to truth".
So that both the gospel and Christ must be His, to whom appertain the law and the nature which are to be vindicated by the gospel and Christ-even at that judgment of God which, as he previously said, was to be according to truth.