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1 John 1:10 (WEB)
If we say that we haven’t sinned, we make him a liar, and his word is not in us.

Catechism · 3

§2147Article 2 · The Second CommandmentRead at vatican.va
“To be unfaithful to them is to misuse God's name and in some way to make God out to be a liar.” — Catechism footnote: Cf. 1 Jn 1:10.

§2631Article 3 · In the Age of the ChurchRead at vatican.va
The first movement of the prayer of petition is asking forgiveness, like the tax collector in the parable: "God, be merciful to me a sinner!"

§827Paragraph 3 · The Church is One, Holy, Catholic, and ApostolicRead at vatican.va
“All members of the Church, including her ministers, must acknowledge that they are sinners.” — Catechism footnote: Cf. 1 Jn 1:8-10.

John Cassian, Conferences XI–XVII XI.9Conference XI
“…we have no sin we deceive ourselves and the truth is not in us;” and again: “If we say that we have not sinned, we make Him a liar, and His word is not in us.”” — editor's note: 1 John i. 8, 10.
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John Cassian, Conferences XVIII–XXIV XX.12Conference XX
“Since it was of these that one of the disciples, whom Jesus loved, declared and laid down absolutely saying: “If we say that we have no sin we deceive ourselves, and His word is not in us.”” — editor's note: 1 John i. 8, 10.
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Augustine of Hippo, Homilies on the First Epistle of John Homily 1
“If we say that we have not sinned, we make Him a liar, and His word is not in us.”” — editor's note: 1 John i. 9, 10.
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Augustine of Hippo, Of Holy Virginity Chapter 49
“But if we shall have said that we have not sinned, we shall make Him a liar, and His word shall not be in us.”” — editor's note: 1 John i. 8–10. [See R.V.]
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