Internal testimony of the Spirit (testimonium Spiritus sancti internum): The working of the Holy Spirit in bringing about human confidence in the truthfulness of Scripture regarding God's promises of salvation to all who exercise faith in Christ. The Word and the Spirit work together as a single testimony but in two manifestations: internally and externally. As the Scriptures are read, testifying to the work of Christ in bringing about salvation to those who believe (external), the Holy Spirit works within the believer to give a spiritual sense of the reality of faith (internal). The doctrine of the internal testimony has its origins in Augustine and was especially affirmed by Protestant theologians during and after the Reformation.1
1. Stanley J. Grenz, David Guretzki & Cherith Fee Nordling, Pocket Dictionary of Theological Terms (Downers Grove, IL: InterVarsity Press, 1999), p. 67.