The Spirit of God is the Spirit of truth, because He speaks the truth, especially about Christ.[1] John gives us two certainties about the Spirit of God.
Christology: The Spirit testifies of the true Christ. In every age, Satan fights against the knowledge of Him, because it is by trusting in Him that sinners are snatched out of his lies and led unto eternal salvation. John focuses on the incarnation, because this was the doctrine heavily under attack in his day. The incarnation is important, because 1) it is the means by which God with finality revealed Himself (John 1:14, 18; Heb. 1:2), 2) it is required for atonement for sin (Heb. 2:17) and therefore redemption,[2] and 3) it is the body with which Jesus will return, with whom we will spend all eternity.[3] Holding fast to the incarnate Christ, therefore, has eternal consequences.[4] Incarnation is just one aspect of the true Christ; we cling onto the whole Christ found in Scripture.
Scripture: The Spirit testifies of Him through Scripture, the enduring witness of the Apostles (4:6). The Spirit’s truth is not discerned by feelings or our own subjective experiences but by the objective revelation of Scripture. Thus, the church in every age must hold fast to it. The Bible is the rope that fastens us to the true Savior, the anchor. May the Spirit keep us from drifting to a Jesus of our own making, a pseudo savior who is powerless to save from sin and hell.[5]
[1] Jesus repeatedly called the Spirit “the Spirit of truth” for this very reason (John 14:17; 15:26; 16:13).
[2] Jesus affirmed this with the language of His coming. The Son of God came to earth to accomplish the will of the Father, to redeem His people (John 6:38-39; 12:46; 18:37).
[3] We believe in Him who is real and physical, who will come in the same manner as He went up (Acts 1:11), whom we will see (3:2), whom we will meet in the air, with whom we will forever be (1 Thess. 4:17).
[4] True believers love Jesus’ appearing (2 Tim. 4:8), both His first and His second coming (Phil. 3:20; Tit. 2:13).
[5] Jesus saves (Luke 19:10; John 3:17; Rev. 1:5-7) those who trust and follow Him (Matt. 7:21-23; John 10:27).