1 John 5:21

Filioli, custodite vos a simulacris (Vulg.). The exhortation arises naturally. “This” this God revealed and made near and sure in Christ “is the True God and Life Eternal. Cleave to Him, and do not take to do with false Gods: guard yourselves from the idols.” St. John is thinking, not of the heathen worship of Ephesus Artemis and her Temple, but of the heretical substitutes for the Christian conception of God. τεκνία gives a tone of tenderness to the exhortation. φυλάσσειν is used of “guarding” a flock (Luke 2:8), a deposit or trust (1 Timothy 6:20; 2 Timothy 1:12; 2 Timothy 1:14), a prisoner (Acts 12:4). φυλάσσειν “watch from within ”; τηρεῖν (see note on 1 John 2:3), “watch from without ”. Thus, when a city is besieged, the garrison φυλάσσουσι, the besiegers τηροῦσιν. The heart is a citadel, and it must be guarded against insidious assailants from without. Not φυλάσσετε, “be on your guard,” but φυλάξατε, aor. marking a crisis. The Cerinthian heresy was a desperate assault demanding a decisive repulse.