“The sum of your word is truth; and every one of your righteous ordinances endures forever,” “and now, O LORD God, you are God, and your words are true”; this is why God’s promises always come true. God is Truth itself, whose words cannot deceive. This is why one can abandon oneself in full trust to the truth and faithfulness of his word in all things. The beginning of sin and of man’s fall was due to a lie of the tempter who induced doubt of God’s word, kindness and faithfulness.
God’s truth is his wisdom, which commands the whole created order and governs the world. God, who alone made heaven and earth, can alone impart true knowledge of every created thing in relation to himself.
God is also truthful when he reveals himself – the teaching that comes from God is “true instruction”. When he sends his Son into the world it will be “to bear witness to the truth”: “We know that the Son of God has come and has given us understanding, to know him who is true.”
From the Catechism of the Catholic Church 215-217