Deuteronomy 4:15

Carefully. Hebrew, "Be therefore particularly attentive, as much as you love your own soul." (Vatable) By keeping my commandments, you can alone obtain salvation, ver. 9. (Menochius)

Similitude of any living creature, such as were the objects of adoration among the pagans. Some represented their gods under the forms of men, women, beasts, birds, or reptiles; while others adored the sun, moon, and stars. (Haydock)

This last was indeed the most ancient species of idolatry, Job xxi. 26. Baal, Astarte, Moloc, Chamos, &c., were different names by which they denoted the heavenly bodies. But the Egyptians carried their superstition to the greatest excess. There was hardly any sort of animal which did not obtain sovereign worship among them. (Calmet)

Their great gods, Isis and Osiris, were sometimes depicted like a man and woman; at other times, like beasts, and frequently they appeared with parts of both. The head of Isis was generally adorned or disfigured with the horns of a bull; (Haydock) and that animal, either alive or in a picture, as well as dogs and cats, were adored throughout the country, while some places had their peculiar idols. The lion, the wolf, and the fish called latus, gave their names to the cities Leontopolis, &c., which had a particular veneration for them. Moses takes care to inform the Hebrews, that the true God is like none of these things; and that they cannot pretend to represent him under any such forms, without doing him an injury. (Calmet)

If Catholics endeavour to put the people in mind of the blessed Trinity, by representing a venerable old man, Jesus Christ in his human nature, and a dove, under which forms the Scripture has introduced the three divine persons, they do not pretend that their divine and most spiritual nature can be thus expressed. "If," says the Council of Trent, Session 25, "the historical accounts of Scripture be sometimes set forth in paintings, for the benefit of the illiterate, let the people be informed that the Divinity is not thus represented, with a design to insinuate that it may be seen with the eyes of the body." So neither can the figure of a triangle, with the ineffable name of God in Hebrew, &c., explain this adorable mystery. But such things may recall to our remembrance, the innumerable benefits which we have received from the three divine persons, after we have been once informed what we have to believe respecting them. This is the laudable motive which has induced the Church to encourage the keeping of such pictures, as well as those of the saints, with due respect. "Not as if we believed that any divinity or virtue resided in them for which they were to be worshipped, or that we should ask any thing of them, or place our confidence in images, as the Gentiles formerly did, who hoped in their idols, (Psalm cxxxiv.,) but because the honour given to them is referred to the originals, which they represent," &c. (Council of Trent, Session 25.) (Haydock)