William Guthrie explains that true saving faith is not merely an intellectual recognition:
I grant, he that believeth on Christ Jesus, believeth what God has said concerning man’s sinful, miserable condition by nature; and he believeth that to be true, that ‘there is life in the Son, who was slain, and is risen again from the dead,’ etc.: but none of these, nor the believing of many such truths, evinces justifying faith, or that believing on the Son of God spoken of in Scripture; for then it were simply an act of the understanding;
The Devil himself has such “faith” (James 2:19). True saving faith requires far more than an acknowledgement that some fact is true.
True saving faith
is chiefly and principally an act or work of the heart and will.
It does entail a particular content, but such content becomes a predicate for a change in the relationship between God and man, it is a reception of Jesus as Savior:
So the Scripture has clearly resolved justifying faith into a receiving of Christ: ‘as many as received Him, to them gave He power to become the sons of God, even to them that believe on His name.’ (John 1: 12.) The receiving of Christ is there explained to be the believing on His name.
The Scripture, to make plain the nature of this reception, uses various images and explains. Thus in Isaiah 26:3, it is calling staying:
3 You keep him in perfect peace whose mind is stayed on you, because he trusts in you. Isaiah 26:3 (ESV)
It is also called “trusting”. It begins with invitation and leads to an unshakeable relationship:
40 For this is the will of my Father, that everyone who looks on the Son and believes in him should have eternal life, and I will raise him up on the last day.” John 6:40 (ESV)
The relationship created supersedes all other concerns:
44 “The kingdom of heaven is like treasure hidden in a field, which a man found and covered up. Then in his joy he goes and sells all that he has and buys that field. 45 “Again, the kingdom of heaven is like a merchant in search of fine pearls, 46 who, on finding one pearl of great value, went and sold all that he had and bought it. Matthew 13:44–46 (ESV)