Referring to the heretical substitutes for the true God that were brought in by the Gnostics and the Cerinthians, as revealed in this Epistle and in John's Gospel and as referred to in the preceding verse. The idols here also refer to anything that replaces the real God. As genuine children of the genuine God, we should be on the alert to guard ourselves from these heretical substitutes and from all vain replacements for our genuine and real God, with whom we are organically one and who is eternal life to us. This is the aged apostle's word of warning to all his little children as a conclusion to his Epistle.
The center of this Epistle's revelation is the divine fellowship of the divine life, the fellowship between the children of God and their Father God, who is not only the source of the divine life but is also light and love as the source of the enjoyment of the divine life (1 John 1:1-7; 4:8, 16). To enjoy the divine life we need to abide in its fellowship according to the divine anointing (1 John 2:12-28; 3:24), based on the divine birth with the divine seed for the development of the divine birth (1 John 2:29; 3:1-10). This divine birth was carried out by three means: the terminating water, the redeeming blood, and the germinating Spirit (vv. 1-13). By these we have been born of God to be His children, possessing His divine life and partaking of His divine nature (1 John 2:29; 3:1). He is now indwelling us through His Spirit (1 John 3:24; 4:4, 13) to be our life and life supply that we may grow with His divine element unto His likeness at His manifestation (1 John 3:1-2). To abide in the divine fellowship of the divine life, that is, to abide in the Lord (1 John 2:6; 3:6), is to enjoy all His divine riches. By such abiding we walk in the divine light (1 John 1:5-7) and practice the truth, righteousness, love, the will of God, and the commandments of God (1 John 1:6; 5, 2:17, 2:29; 3:10-11; 5:2) by the divine life received through the divine birth (1 John 2:29; 4:7). To preserve ourselves in this abiding in the divine fellowship, three main negative things need to be dealt with. The first is sin, which is lawlessness and unrighteousness (1 John 1:7-10; 2:1-6; 3:4-10; 5:16-18); the second is the world, which is composed of the lust of the flesh, the lust of the eyes, and the vainglory of this life (1 John 2:15-17; 4:3-5; 5:4-5, 19); and the last is idols, which are the heretical substitutes for the genuine God and the vain replacements for the real God (v. 21). These three categories of exceedingly evil things are weapons used by the evil one, the devil, to frustrate, harm, and, if possible, even annihilate our abiding in the divine fellowship. The safeguard against his evildoings is our divine birth with the divine life (v. 18); and, based on the fact that through His death on the cross the Son of God destroyed the works of the devil (1 John 3:8), we overcome him by the word of God that abides in us (1 John 2:14). By virtue of our divine birth we also overcome his evil world by our faith in the Son of God (vv. 4-5). Moreover, our divine birth with the divine seed that was sown into our inner being enables us not to live habitually in sin (1 John 3:9; 5:18), because Christ has taken away sins through His death in the flesh (1 John 3:5). In case we sin occasionally, we have our Advocate as our propitiatory sacrifice to take care of our case before our Father God (1 John 2:1-2), and His everlasting efficacious blood cleanses us (1 John 1:7). Such a revelation is the basic and substantial element of the apostle's mending ministry.