Some comparisons work in both directions, as: the black box is like the brown box; the brown box is like the black box. Others do not, as: the self-portrait is like the artist; but not, the artist is like the self-portrait.
Aquinas explains, in reference to man’s likeness to God in contrast to God’s unlikeness to man:
Further, among like things there is mutual likeness; for like is like to like. If therefore any creature is like God, God will be like some creature, which is against what is said by Isaias: “To whom have you likened God?” (Isaiah 40:18). (ST I, Q. 4, Art. 3, arg. 4)[…]
Although it may be admitted that creatures are in some sort like God, it must nowise be admitted that God is like creatures; because, as Dionysius says (Div. Nom. ix): “A mutual likeness may be found between things of the same order, but not between a cause and that which is caused.” For, we say that a statue is like a man, but not conversely; so also a creature can be spoken of as in some sort like God; but not that God is like a creature. (ST I, Q. 4, Art. 3, ad. 4)
One could expand upon this: what Aquinas sets out is the true nature of things, the ontological real McCoy (as it were). But notice how idolatry reverses this relation:
The idols of the nations are silver and gold,
the work of human hands.
They have mouths, but do not speak;
they have eyes, but do not see;
they have ears, but do not hear,
nor is there any breath in their mouths.
Those who make them become like them,
so do all who trust in them. (Psalm 135:15–18)
When the proper way of relating the creature and the things made by that creature to the Creator of all things is lost, distorted, perverted, an unseemly and ridiculous wickedness follows, in which we act in sin against the grain of the way the world really is. Idolatry is a fixation on unreality. And, unlike in the second kind of comparison discussed above, one could add: a fixation on unreality is idolatry.