Et in hoc scimus quoniam cognovimus eum si mandata eius observemus qui dicit se nosse eum et mandata eius non custodit mendax est in hoc veritas non est qui autem servat verbum eius vere in hoc caritas Dei perfecta est in hoc scimus quoniam in ipso sumus.
Καὶ ἐν τούτῳ γινώσκομεν ὅτι ἐγνώκαμεν αὐτόν, ἐὰν τὰς ἐντολὰς αὐτοῦ τηρῶμεν. ὁ λέγων ὅτι Ἔγνωκα αὐτὸν καὶ τὰς ἐντολὰς αὐτοῦ μὴ τηρῶν ψεύστης ἐστίν, καὶ ἐν τούτῳ ἡ ἀλήθεια οὐκ ἔστιν· ὃς δ’ ἂν τηρῇ αὐτοῦ τὸν λόγον, ἀληθῶς ἐν τούτῳ ἡ ἀγάπη τοῦ θεοῦ τετελείωται. ἐν τούτῳ γινώσκομεν ὅτι ἐν αὐτῷ ἐσμεν·
We can be sure that we know God only by keeping his commandments. Anyone who says, ‘I know him’, and does not keep his commandments, is a liar, refusing to admit the truth. But when anyone does obey what he has said, God’s love comes to perfection in him. (2:3-5)
First Epistle of St. John
A lie [mendacium] may be in itself contrary to charity by reason of its false signification. For if this be about divine things, it is contrary to the charity of God, whose truth one hides or corrupts by such a lie; so that a lie of this kind is opposed not only to the virtue of charity, but also to the virtues of faith and religion: wherefore it is a most grievous and a mortal sin. [IIa-IIae q. 110 a. 4 co.]
St Thomas Aquinas, Summa Theologiae
n., the tendency to be untruthful
[from Late Latin mendācitās, from Latin mendāx untruthful]