"The person who knows the truth of the promise only intellectually, without clinging to it, does not believe savingly.
When the devil tempted Christ he did not dispute against Scripture, but from Scripture, drawing his arrows out of this very quiver Matthew 4:6. And at another time, he makes as full a confession of Christ as Peter himself did Matthew 8:29, 16:17. Assent to the truth of the Word is but an act of the understanding, which reprobates and devils may exercise. But justifying faith has its substance both in the understanding and the will; therefore it is called a believing "with the heart" Romans 10:10. Philip said, "If you believe with all of your heart, you may". Acts 8:37. It takes in all the powers of the soul.
There is a double object in the promise, which relates to both the understanding and the will. As the promise is true, so it calls for an act of assent from the understanding; and as it is good as well as true, so it calls for an act of the will to embrace it. Therefore, the person who knows the truth of the promise only intellectually, without clinging to it, does not believe savingly. That man no more receives benefit from the promise than a person who realizes food is nourishing but refuses to eat.
Justifying faith is not assurance. If it were, John might have spared himself the trouble of writing to "you that believed on the name of the Son of God, that ye might know that ye have eternal life" 1 John 5:13. His readers might then have said, "We already do this. Is it not faith to believe that we are among those pardoned through Christ, and that we shall be saved through Him?" But this cannot be so. If faith were assurance, then a man's sins would be pardoned before he believes, for surely he must be pardoned before he can know he is pardoned. The candle must be lighted before I can see it is lighted. The child must be born before I can be assured it is born. The object must be before the act. ”
Quoted material from, ”The Christian in Complete Armour Daily Readings in Spiritual Warfare” by Gurnall and James S Bell. http://www.moodypublishers.com/pub_productDetail.aspx?id=41823&pid=53617