The Threefold Method of Assurance

"When the Spirit of truth comes," Jesus declared on the eve of his betrayal and arrest, "he will glorify me, for he will take what is mine and declare it to you." The Holy Spirit, then, fulfills his role as Comforter by pointing believers not to himself but to Christ, thereby providing them assurance of salvation.

But how does the Spirit do this?

In no other discussion is the "shy member" of the Trinity's bashfulness as obvious as in that of his role as witness-bearer to our status as adopted sons of God the Father. Many unwittingly assign the Spirit the job of divine whisperer whose greatest function is, in the midst of the believer's quiet time, to say in his still, small voice, "Pssst! You’re elect!"

It is common in Reformed circles to speak of the "threefold method of assurance," referring to the comfort we derive, first, from the divine promises found in Scripture, secondly, from the fruit of sanctification in our lives, and thirdly, through the witness of the Holy Spirit (this method's proponents include Theodore Beza, William Perkins, Willem Teelinck, Thomas Goodwin, John Owen, and Martyn Lloyd-Jones).

What are we to make of this view of assurance? How does the Spirit "bear witness with our spirits that we are children of God"?