A witness presupposes a person, object, content, or event concerning which testimony is given. The NT makes it clear that the primary witness of the Spirit is to Christ, and not to Himself or initially to a body of doctrine (John 14:26; 15:26; 16:7-15; cf. Matt 16:16, 17; 1 John 2:20-22). Though the Spirit’s witness focuses upon the person and work of Christ, it moves out from that central point to incorporate (a) the totality of God’s saving acts for man, (b) the intrinsic and instrumental authority of Scripture, (c) the nature of fallen man and his response to God, and (d) a ministry of assurance and instruction to God’s own.
A. Kuyper, The Work of the Holy Spirit (1900); H. B. Swete, The Holy Spirit in the Ancient Church (1912); C. W. Hodge, “The Witness of the Holy Spirit to the Bible,” PTR, XI (1913), 41-84; B. Ramm, The Witness of the Spirit (1960).
International Standard Bible Encyclopedia (1915)
This phrase arises from the words of Ro 8:16: "The Spirit himself beareth witness with our spirit, that we are children of God." With this may be grouped, as illustrative, 1 Joh 5:10: "he that believeth on the Son of God hath the witness in him." in interpreting, we may approach the former passage through the latter. To the man who "believeth on the Son of God," so as to prove him by reliance, He becomes self-evidential in experience, verifying himself to the believer as the divine response to his whole spiritual need. Thus, believed on as the Son, he awakens in the soul which he embraces the filial attitude toward God, the cry, "Abba, Father." On the other side the Spirit, both in the written Word (e.g. Joh 1:12) and in his secret converse with the believer in the life of faith, assures him of the paternal love toward him, as toward a "dear child," (Eph 5:1) of the Father of his Lord. There is thus a concurrent "witnessing." The believer’s spirit says, "Thou art my Father"; the Spirit, says to the believer’s spirit, "Thou art His child." We may compare Ro 5:5: "The love of God hath been shed abroad in our hearts through the Holy Spirit."