The Catechism makes clear that the Eighth Commandment is fulfilled by living the Truth.

2465   The Old Testament attests that God is the source of all truth. His Word is truth. His Law is truth. His "faithfulness endures to all generations." Since God is "true," the members of his people are called to live in the truth.

2466   In Jesus Christ, the whole of God's truth has been made manifest. "Full of grace and truth," he came as the "light of the world," he is the Truth. "Whoever believes in me may not remain in darkness." The disciple of Jesus continues in his word so as to know "the truth [that] will make you free" and that sanctifies. To follow Jesus is to live in "the Spirit of truth," whom the Father sends in his name and who leads "into all the truth." To his disciples Jesus teaches the unconditional love of truth: "Let what you say be simply 'Yes or No.'"

2467   Man tends by nature toward the truth. He is obliged to honor and bear witness to it: "It is in accordance with their dignity that all men, because they are persons . . . are both impelled by their nature and bound by a moral obligation to seek the truth, especially religious truth. They are also bound to adhere to the truth once they come to know it and direct their whole lives in accordance with the demands of truth."

2468   Truth as uprightness in human action and speech is called truthfulness, sincerity, or candor. Truth or truthfulness is the virtue which consists in showing oneself true in deeds and truthful in words, and in guarding against duplicity, dissimulation, and hypocrisy.

2469   "Men could not live with one another if there were not mutual confidence that they were being truthful to one another." The virtue of truth gives another his just due. Truthfulness keeps to the just mean between what ought to be expressed and what ought to be kept secret: it entails honesty and discretion. In justice, "as a matter of honor, one man owes it to another to manifest the truth."

2470   The disciple of Christ consents to "live in the truth," that is, in the simplicity of a life in conformity with the Lord's example, abiding in his truth. "If we say we have fellowship with him while we walk in darkness, we lie and do not live according to the truth."

Catechism of the Catholic Church

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