It is the sovereign prerogative of Christ's spirit to convince men's consciences of the truth of Christ's gospel; and Christ's human witnesses must learn to ground their hopes of success not on clever presentation of the truth by man, but on powerful demonstration of the truth by the Spirit

Knowing God, Packer, pg 71

He (Holy Spirit) testifies by illuminating: opening blinded eyes, restoring spiritual vision, enabling sinners to see that the gospel is indeed God's truth, and Scripture is indeed God's Word, and Christ is indeed God's son

Knowing God, Packer, pg 71