Calvin's Commentaries, Vol. 4: Harmony of the Law, Part II, tr. by John King, [1847-50], at sacred-texts.com
21. Thou shalt not plant thee a grove of any trees near unto the altar of the LORD thy God, which thou shalt make thee.
21. Non plantabis tibi lucum quarumvis arborum apud altare Jehovae Dei tui, quod facies tibi.
21. Thou shalt not plant thee. It is plain from the end of this verse that it is part of the Second Commandment. We know 300 that amongst the heathen nations groves were sacred, so that with them no religious object would receive due reverence, except under the shade of trees. Wherefore lest conformity with this general custom should vitiate the pure worship of God, this distinction was made; and this then is the intent of the prohibition, that the Jews should fly from all strange rites, lest by too closely approaching the Gentiles, they should introduce a sinful medley. But how necessary this prohibition was, appears from their eager imitation (of the heathen), of which mention is constantly made in the sacred history. For there was scarcely any period in which they abstained from “high places.” Nor is it without reason that Isaiah and Jeremiah reprove them for “playing the harlot under every green tree.” (Isa 57:5; Jer. 2:20, Jer. 3:6.)
See Lucian in Dea Syria, sub initium; Πρῶτοι μὲν ὦν ἀνθρώπων, τῶν ἡμεῖς ἴδμεν, Αἰγύπτιοι λέγονται θεῶν τε ἐννοίαν λαβεὶν, καὶ ἱρὰ εἴσασθαι, καὶ τεμένεα, κ. τ. λ., — Tacitus Germ. 9. “Lucos et nemora consecrant.” Virgil. Eccl. 6:73. “Ne quis sit lucus, quo plus se jactet Apollo."