It says that "they are disposed to that justice". You need to be disposed to receive justification by the sacrament of baptism. Hearing does not transmit the faith, hearing disposes to be prepared to receive the faith.
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But what it
says is that the disposition to justice consists in faith (a grace "received by hearing" whereby man believes all which God has revealed), hope, and charity (which culminates in a
resolve for baptism and a commitment to abandon the old man and follow Christ's commandments).
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It says nothing about the reception of water baptism as a required element in the disposition for justification. Baptism is the instrumental cause, as one reads in the next paragraph. The disposition to justice
itself requires supernatural faith. I am not sure how one can read Trent any other way. Here, let's look at the full paragraph (this more or less immediately follows Trent's teaching that justification cannot be effected without baptism or a desire for it, and it immediately precedes Trent's teachings on the cause of justification):
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Now they are disposed to that justice [can. 7 and 9] when, aroused and assisted by divine grace, receiving faith "by hearing" [Rom. 10:17], they are freely moved toward God, believing that to be true which has been divinely revealed and promised [can. 12 and 14], and this especially, that the sinner is justified by God through his grace, "through the redemption which is in Christ Jesus" [Rom. 3:24], and when knowing that they are sinners, turning themselves away from the fear of divine justice, by which they are profitably aroused [can. 8], to a consideration of the mercy of God, they are raised to hope, trusting that God will be merciful to them for the sake of Christ, and they begin to love him as the source of all justice and are therefore moved against sins by a certain hatred and detestation [can. 9], that is, by that repentance, which must be performed before baptism [Acts 2:38]; and finally when they resolve to receive baptism, to begin a new life and to keep the commandments of God. Concerning this disposition it is written: "He that cometh to God must believe, that he is and is a rewarder to them that seek him" [Heb. 11:6], and, "Be of good faith, son, thy sins are forgiven thee" [Matt. 9:2; Mark 2:5], and, "The fear of the Lord driveth out sin" [Sirach. 1:27], and, "Do penance, and be baptized every one of you in the name of Jesus Christ for the remission of your sins, and you shall receive the Holy Spirit" [Acts 2:38], and, "Going therefore teach all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all things whatsoever I have commanded you" [Matt. 28:19], and finally, "Prepare your hearts unto the Lord" [1 Samuel 7:3]. (Denz. 798 )
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I emphasized a few things for convenience. Mainly, that Trent is describing the acquisition of
supernatural faith on the part of the catechumen. As one continues reading, it's very clear that this paragraph is saying much more than "catechumens need to learn what's in the faith." As you just pointed out, this paragraph concerns the preparation of the sinner for restoration to justice. That is correct, and that preparation
consists in the acquisition of all three virtues, the first of which is supernatural faith, the greatest of which is supernatural charity.
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Your contention was that supernatural faith is only first ever received through baptism. If that were true, Trent would not describe its reception in the Catechumen altogether prior to baptism. But it does. If you have a new argument that's fine. But right now I'm only concerned with your claim that faith is only first received at baptism. Trent clearly teaches otherwise, as we see here.