John Calvin Commentary 1 John 5:21

"[My] little children, guard yourselves from idols." — 1 John 5:21 (ASV)

Keep yourselves from idols. Although this is a separate sentence, it is, so to speak, an appendix to the preceding doctrine. For the life-giving light of the Gospel should scatter and dissipate not only darkness but also all mists from the minds of the godly. The Apostle not only condemns idolatry but commands us to beware of all images and idols, by which he implies that the worship of God cannot continue uncorrupted and pure whenever people begin to be captivated by idols or images.

For superstition is so innate in us that the slightest opportunity will infect us with its contagion. Dry wood will not burn as easily when coals are put under it as idolatry will seize and engross the minds of people when an opportunity is given to them. And who does not see that images are the sparks? Sparks, do I say? No, rather torches, which are sufficient to set the whole world on fire.

The Apostle at the same time not only speaks of statues but also of altars, and includes all the instruments of superstitions. Moreover, the Papists are ridiculous, who pervert this passage and apply it to the statues of Jupiter and Mercury and the like, as if the Apostle did not generally teach that there is a corruption of religion whenever a corporeal form is ascribed to God, or whenever statues and pictures form a part of His worship.

Let us then remember that we should carefully continue in the spiritual worship of God, so that we banish far from us everything that might turn us aside to gross and carnal superstitions.