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Romans 6:10 (WEB)
For the death that he died, he died to sin one time; but the life that he lives, he lives to God.

Catechism · 2

§1085Article 1 · The Liturgy - Work of the Holy TrinityRead at vatican.va
“When his Hour comes, he lives out the unique event of history which does not pass away: Jesus dies, is buried, rises from the dead, and is seated at the right hand of the Father "once for all."” — Catechism footnote: Rom 6:10; Heb 7:27; 9:12; cf. Jn 13:1; 17:1.

§1987Article 2 · Grace and JustificationRead at vatican.va
“So you also must consider yourselves as dead to sin and alive to God in Christ Jesus.” — Catechism footnote: Rom 6:8-11.

Origen, Against Celsus II.68Book II
“…the world is crucified to me, and I unto the world;” and as His death was necessary, because of the statement, “For in that He died, He died unto sin once,” and this, “Being made conformable to His death,” and this, “For if we be dead with Him, we shall also live with Him:” so also His burial has an application to…” — editor's note: Rom. vi. 10.
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Gregory of Nyssa, Against Eunomius VI.1Book VI
“…“yet liveth by power” (while he indicates by “power” the Divine Nature); and, “He died unto sin” (that is, with regard to the body), “but liveth unto God” (that is, with regard to the Godhead, so that by these words it is established that, while the Man tasted death, the immortal Nature did not admit the…” — editor's note: Rom. vi. 10.
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Gregory of Nyssa, The Great Catechism Chapter 37
“But had it been possible for him in his imitation to undergo a complete dying, the result would be not imitation but identity; and the evil of our nature would so entirely vanish that, as the Apostle says, “he would die unto sin once for all.”” — editor's note: ἐφάπαξ. So Rom. vi. 10, “He died unto sin once” (A.V.); i.e. once for all.
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Ambrose of Milan, Letters Letter 7
“Peter understood the divine answer to refer to his own cross, for Christ could not be crucified a second time, for He had put off the flesh by the passion of the death which He had undergone; since: “In that He died, He died unto sin once, but in that He liveth, He liveth unto God.”” — editor's note: Rom. vi. 10.
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Tertullian, On Modesty Chapter 17
“Thus, too, repute ye yourselves dead indeed to sin, but living to God through Christ Jesus.”” — editor's note: Rom. vi. 1–11.
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Hilary of Poitiers, On the Trinity Book 9
“Even so reckon ye yourselves also to be dead unto sin, but alive unto God in Christ Jesus, ascribing His death to sin, that is, to our body, but His life to God, Whose nature it is to live.” — editor's note: Rom. vi. 10, 11.
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Augustine of Hippo, The Enchiridion Chapter 52
“To those who are baptized into the death of Christ, then,—and this class includes not adults only, but infants as well,—he says: “Likewise reckon ye also yourselves to be dead indeed unto sin, but alive unto God through Jesus Christ our Lord.”” — editor's note: Rom. vi. 1–11
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