Musar for Shabbat 109:12

And our sages, of blessed memory, said: "the seal of the Holy One, blessed be He, is truth" (Shab.55a). If "truth" is what the Holy One, blessed be He, selected as His seal, then undoubtedly, how great of an abomination must its opposite be to Him!

God made man to be upright (see Eccl. 7:29), and the seal of the Holy One, Blessed be He, is Truth (Shabbath 55a and see T.P. Sanh. 1:5). And it is written, "He that speaketh falsehood shall not be established before mine eyes" (Ps. 101:7). When a man occupies himself with falsehood, then the falsehood does not cleave to the truth. And where there is Truth is as though one were able to describe it as the place of His dwelling in the heavens and directed towards mankind, for where there is Truth among mankind, then everyone concedes that He made heaven and earth and the sea and all they contain. And this is what is said, "Who made heaven and earth, the sea and all that in them is" (Ps. 146:6). And this is followed by, "Who keepeth truth for ever" (ibid.). But where there is treachery and falsehood, it would appear that the place of the dwelling of the Eternal One is not in the heaven and the earth. And he who is worthy to consider these things, how the souls are hewn out from the source of Truth, will conduct all of his affairs with Truth, and he will not permit falsehood to enter into the place of the holiness of truth. And therefore the verse says, "The Lord is nigh unto all them that call upon Him in truth" (Ps. 145:18). For the Holy One, Blessed be He, who is Truth, draws near to him who calls upon Him in truth. And what is meant by, "Who call upon Him in truth?" This refers to the one who cleanses his heart of everything in the world, and draws near to the Holy One, Blessed be He, alone; and whose mind continually strengthens itself to cleave to the supernal light; and who directs his thought in that desire always. And this is what a man can attain when he sits alone in his room and studies, and for this reason every pious person should often separate himself and sit alone, and should not associate with people, except for a great need. But if he prays with only the movement of his lips, facing the wall, and thinks about the affairs of the world, and he calls to God with his tongue, while his mind is on some thing else, or if he expects to be honored because of the sweetness of his voice, and he tries to find favor in the eyes of people, in order to receive praise from them, then his service is not true. And he is one of those of whom it is said, "Thou art near in their mouth, and far from their reins" (Jer. 12:2).