This means that Jesus Christ brought not some more or less detailed and clear understanding of grace and truth, but the grace itself, the truth itself, essentially bestowed upon people, instilled in people. We have been made partakers of the divine nature (2 Pet. 1:4).
Truth has a characteristic Spirit. This spirit is called the Spirit of Truth (cf. Jn. 15:26; 16:13).
It is Spirit, proceeding from the Father (cf. Jn. 15:26). It is the Holy Spirit of God (cf. Jn. 14:26). It is the Spirit of the Son (cf. Gal. 4:6), as inseparably close to the Son, as comprising together with the Father and the Son one undivided and unmingled Divine Essence.
Accepting the Truth, we also accept the Holy Spirit—that is why the All-Holy Truth says of Himself, that He will send the Holy Spirit from the Father to His disciples.
Naturally, the Holy Spirit of Truth will be present where Holy Truth acts, and will leave the effect of its action.
In like manner, where the Holy Spirit works, there will be an abundant manifestation of Truth, as the Lord also said to His disciples: Howbeit when he, the Spirit of truth, is come, he will guide you into all truth (Jn. 16:13).
Describing the wondrous relationship of the Divine Word to the Divine Spirit, the Lord said of the Spirit: He shall glorify me: for he shall receive of mine, and shall shew it unto you. All things that the Father hath are mine (Jn. 16:14–15).
The Spirit shows and manifests to people the Son co-natural to Him. The Holy Spirit spiritually forms the true Christian and transforms him into a dwelling place of God (cf. Eph. 2:22).
He represents Christ and instills Him in the inner man (cf. Eph. 3:16–17). He makes people God’s children by adoption, making them like unto Christ, establishing Christ-like qualities in them (cf. Jn. 14:6).
People who have been made children of God by adoption turn to Him in their prayers as to their Father, because the Holy Spirit very clearly and tangibly witnesses to the spirit of a person renewed by Him (cf. Rom 8:16) concerning that person’s union with God, his adoption by God.
And because ye are sons, says the Apostle, God hath sent forth the Spirit of his Son into your hearts, crying, Abba, Father (Gal. 4:6).
Such worshippers are recognized as true worshippers of God! Such worshippers, who worship God in Spirit and in Truth, seek and receive God.
There is no knowledge of God outside of true Christianity, and no service of God.
Ignatius Brianchaninov (1807–1867; Russian Orthodox): Homily on the Sunday of the Samaritan Women on Worshipping God in Spirit and in Truth translated by Nun Cornelia Rees @ Pravoslavie.