The Greek word means reality (the opposite of vanity), verity, veracity, genuineness, sincerity. It is John's highly individual terminology, and it is one of the profound words in the New Testament, denoting all the realities of the divine economy as the content of the divine revelation, conveyed and disclosed by the holy Word as follows:
1) God, who is light and love, incarnated to be the reality of the divine things, such as the divine life, the divine nature, the divine power, and the divine glory, for us to possess, that we may enjoy Him as grace, as revealed in John's Gospel (John 1:1, 4, 14-17).
2) Christ, who is God incarnated and in whom all the fullness of the Godhead dwells bodily (Col. 2:9), as the reality of
a) God and man (John 1:18, 51; 1 Tim. 2:5);
b) all the types, figures, and shadows of the Old Testament (Col. 2:16-17; John 4:23, 24, and notes);
c) all the divine and spiritual things, such as the divine life and resurrection (John 11:25; 14:6), the divine light (John 8:12; 9:5), the divine way (John 14:6), wisdom, righteousness, sanctification, and redemption (1 Cor. 1:30).
Hence, Christ is the reality (John 14:6; Eph. 4:21).
3) The Spirit, who is Christ transfigured (1 Cor. 15:45b; 2 Cor. 3:17), as the reality of Christ (John 14:16-17; 15:26) and of the divine revelation (John 16:13-15). Hence, the Spirit is the reality (1 John 5:6).
4) The Word of God as the divine revelation, which not only reveals but also conveys the reality of God and Christ and of all the divine and spiritual things. Hence, the Word of God also is reality (John 17:17 and note John 17:173).
5) The contents of the faith (belief), which are the substantial elements of what we believe, as the reality of the full gospel (Eph. 1:13; Col. 1:5); these are revealed throughout the New Testament (2 Cor. 4:2; 13:8; Gal. 5:7; 1 Tim. 1:1, note 1, points 1 and 2; 1 Tim. 2:4 and note 2; 1 Tim. 2:7b; 1 Tim. 3:15 and note 5; 1 Tim. 4:3; 1 Tim. 6:5; 2 Tim. 2:15, 18, 25; 3:7, 8; 4:4; Titus 1:1, 14; 2 Thes. 2:10, 12; Heb. 10:26; James 5:19; 1 Pet. 1:22; 2 Pet. 1:12).
6) The reality concerning God, the universe, man, man's relationship with God and with his fellow man, and man's obligation to God, as revealed through creation and the Scriptures (Rom. 1:18-20; 2:2, 8, 20).
7) The genuineness, truthfulness, sincerity, honesty, trustworthiness, and faithfulness of God as a divine virtue (Rom. 3:7; 15:8) and of man as a human virtue (Mark 12:14; 2 Cor. 11:10; Phil. 1:18; 1 John 3:18), and as an issue of the divine reality (John 4:23-24; 2 John 1:1a; 3 John 1:1).
8) Things that are true or real, the true or real state of affairs (facts), reality, veracity, as the opposite of falsehood, deception, dissimulation, hypocrisy, and error (Mark 5:33; 12:32; Luke 4:25; John 16:7; Acts 4:27; 10:34; 26:25; Rom. 1:25; 9:1; 2 Cor. 6:7; 7:14; 12:6; Col. 1:6; 1 Tim. 2:7a).
Of the eight points listed above, the first five refer to the same reality in essence. God, Christ, and the Spirit — the Divine Trinity — are essentially one. Hence, these three, being the basic elements of the substance of the divine reality, are actually one reality. This one divine reality is the substance of the Word of God as the divine revelation. Hence, it becomes the revealed divine reality in the divine Word and makes the divine Word the reality. The divine Word conveys this one divine reality as the contents of the faith, and the contents of the faith are the substance of the gospel revealed in the entire New Testament as its reality, which is just the divine reality of the Divine Trinity. When this divine reality is partaken of and enjoyed by us, it becomes our genuineness, sincerity, honesty, and trustworthiness as an excellent virtue in our behavior that enables us to express God, the God of reality, by whom we live; and we become persons living a life of truth, without any falsehood or hypocrisy, a life that corresponds with the truth revealed through creation and the Scripture.
The word truth is used in the New Testament more than one hundred times. Its denotation in each occurrence is determined by its context. For instance, in John 3:21, according to the context, it denotes uprightness (the opposite of evil — John 3:19-20), which is the reality manifested in a man who lives in God according to what He is and which corresponds with the divine light, which is God, as the source of the truth, manifested in Christ. In John 4:23-24, according to the context of that chapter and also according to the entire revelation of John's Gospel, it denotes the divine reality becoming man's genuineness and sincerity (the opposite of the hypocrisy of the immoral Samaritan worshipper — John 4:16-18) for the true worship of God. The divine reality is Christ, who is the reality (John 14:6), as the reality of all the offerings of the Old Testament for the worship of God (John 1:29; 3:14) and as the fountain of living water, the life-giving Spirit (John 4:7-15), partaken of and drunk by His believers to be the reality within them, which eventually becomes their genuineness and sincerity, in which they worship God with the worship that He seeks. In John 5:33; 18:37, according to the entire revelation of the Gospel of John, truth denotes the divine reality embodied, revealed, and expressed in Christ as the Son of God. In John 8:32, 40, 44-46, according to the context of that chapter, it denotes the reality of God revealed in His word (John 8:47) and embodied in Christ, the Son of God (John 8:36), which sets us free from the bondage of sin (see note John 8:321a).
Here in v. 6, truth denotes the revealed reality of God in its aspect of the divine light. It is the issue and realization of the divine light mentioned in v. 5. The divine light is the source in God; truth is the issue and realization of the divine light in us (see note 1 John 4:82c; cf. John 3:19-21). When we abide in the divine light, which we enjoy in the fellowship of the divine life, we practice the truth — what we have realized in the divine light. When we abide in the source, its issue becomes our practice.