Jamieson, Fausset and Brown on Isaiah 66:17

17. in ... gardens--Hebrew and the Septuagint rather require, "for (entering into) gardens," namely, to sacrifice there [Maurer].

behind one tree--rather, "following one," that is, some idol or other, which, from contempt, he does not name [Maurer]. Vitringa, &c., think the Hebrew for "one," Ahhadh, to be the name of the god; called Adad (meaning One) in Syria (compare Ac 17:23). The idol's power was represented by inclined rays, as of the sun shining on the earth. Gesenius translates, "following one," namely, Hierophant ("priest"), who led the rest in performing the sacred rites.

in ... midst--namely, of the garden (see on Is 65:3, 4).

mouse--legally unclean (Le 11:29) because it was an idol to the heathen (see on Is 37:36; 1Sa 6:4). Translate, "the field mouse," or "dormouse" [Bochart]. The Pharisees with their self-righteous purifications, and all mere formalists, are included in the same condemnation, described in language taken from the idolatries prevalent in Isaiah's times.