The witness of the Holy Spirit has been all too one-sidedly applied, by Calvin and the later Reformed theologians, to the authority of Holy Scripture. It seemed that it had no other import than the subjective assurance that Scripture is the word of God. As a result this testimony came to stand by itself… Scripture, however, teaches very differently.
Generally speaking, the Holy Spirit was promised by Jesus as the Comforter, the Spirit of truth, who leads first the apostles, then, by their word, also all other believers, into the truth. He witnesses of Christ to them and glorifies him (John 14:17; 15:26; 16:14). To that end he convicts people of sin (John 16:8-11), regenerates them (John 3:3), and prompts them to confess Christ as Lord (1 Cor. 12:3). He further assures them of their adoption as children of God and of their heavenly inheritance (Rom. 8:14f.; 2 Cor. 1:22; 5:5; Eph. 1:13; 4:30), makes known all the things believers have received from God (1 Cor. 2:2; 1 John 2:20;; 3:24; 4:6-13), and in the church is the author of all Christian virtues and all spiritual gifts (Gal. 5:22; 1 Cor. 12:8-11). It is evident from all these passages that the testimony of the Holy Spirit is of a religious-ethical kind and intimately bound up with people’s own faith life. It does not bypass people’s faith; it is not a voice from heaven, a dream or a vision. It is a witness that the Holy Spirit communicates in, with, and through our own spirit in faith. It is not given to unbelievers but is the portion only of the children of God. Episcopius therefore raised the objection that the testimony of the Holy Spirit cannot be a ground of faith because it is something that only comes later (John 7:38; 14:17; Acts 5:32; Gal. 3:2; 4:6). But from the very beginning faith itself is the work of the Holy Spirit (1 Cor. 12:3) and receives its seal and confirmation in the Spirit of adoption. Believing itself is a witness of the Holy Spirit in our hearts and through our spirit.
Reformed Dogmatics Vol. 1: Prolegomena (Grand Rapids, Michigan; Baker Academic; 2003) p. 593-594.