Understanding

"The understanding which the unbeliever lacks can only be provided when his mind has been opened (e.g., Luke 24:45) and he has been convicted by the Spirit of Truth (John 16:8). This Spirit continually witnesses to Christ, conducting His case before the world as Christ's legal representative for the defense (i.e., the "Advocate"; John 15:26). That is, the success of our apologetics depends on the work of the Holy Spirit (cf. John 3:3, 8). Moreover, only if the unbeliever comes to abide in Christ's word can he have God and know the truth (John 8:31-32; 2 John 9). Until he gains the mind of Christ he is completely unable to know Spiritual things (1 Cor. 2:14, 16). Having the mind of Christ requires humility (cf Phil. 2:5, 8)., and thus renunciation of self-sufficiency in order to obey the truth of God. One can only come to a knowledge of Him who is Truth (John 14:6) when the Son grants him the understanding which is lacking (1 John 5:20)."

from "Always Ready" by Dr. Greg L. Bahnsen (p. 85)