Reformed divines generally consider justifying faith to consist of three elements: knowledge (notitia), assent (assensus), and trust (fiducia). A person can have a bare theoretical knowledge of the truths of the Gospel (notitia), and cognitively agree with or assent to them as being true (assensus), both of which are necessary to justifying faith; but it is only when these are accompanied by a turning away from yourself to trust entirely in Christ and his merits alone for salvation (fiducia), i.e. personally applying his merits to yourself by faith, that faith becomes justifying faith. John Edwards (1637-1716) stresses the necessity of this personal application of Christ in his The Doctrin [sic] of Faith and Justification set in a True Light, p. 107-109:
We smile at the Athenian, who being shew’d a map of the world, presently look’d where his house stood, and when he could not find that there, he found fault with the map, as an imperfect representation of the world; for (as he thought) if it had been a complete one, it must needs have had in it his little dwelling at Athens. This, indeed, might argue silliness in the poor man; but apply this to religion, and the business of our souls, and the salvation of them, and then such kind of acting will not be folly, but exceeding great wisdom and prudence. The Holy Scriptures, but especially the Gospel, is the map which we Christians are presented with; it is continually before our eyes, and we are invited to survey the several parts and climates of it. Here is great salvation tendered to us; wherever we cast our eyes, there are manifest discoveries of the love of God in Christ, of his designs of mercy to lost souls, of his glorious purposes to save sinners. But the whole Gospel is no better than an unknown land, to the person that is not particularly interested in it; and therefore that which we are chiefly to mind, is whether we are comprehended in this map of life, and whether besides the general belief of the Gospel, we can particularly apply and appropriate Christ’s purposes of mercy to ourselves. This is the special and peculiar act of justifying faith, and therefore in this we should think ourselves most of all concerned. For as it is with food, physick [i.e. medicine] and apparel, if the first be not eaten, it cannot nourish us; if the second be not taken, it cannot cure us; and if the last be not put on and worn, it cannot warm us: so neither can the mercy of God in Christ be really advantageous to us, unless it be by some proper instrument applied and made use of. The great and precious promises, in which God’s mercies are contain’d and convey’d, are generally propounded to the righteous; but it is a true and operative faith which makes the particular and special application of them to ourselves.
This was represented of old in the Mosaic sacrifices for sin; they were first slain and offer’d, and then the blood of them was sprinkled. This was absolutely necessary, in order to the expiation of sin. Unless those that offer’d the sin-offering had the blood of it sprinkled upon them, they remain’d unpurified. Which occasion’d that of the Psalmist, Purge me with hyssop, and I shall be clean, for the hyssop was made use of in sprinkling the blood. And we find that this sprinkling or application of the blood of the sacrifices is mention’d expressly by the sacred writers of the New Testament, and it is applied to the sufferings of Christ, to let us know, that the shedding of the blood of Christ on the cross will not avail us, except there be added this sprinkling of it upon us, this applying the virtue and merit of his sufferings. And this is done by faith: for by it all things that Christ hath done or suffer’d for us as a Mediator, are applied to us. Him God hath set forth to be a propitiation through faith in his blood, Rom. 3:25. Whence I gather, that it is faith that makes Christ’s undertakings effectual. God is not actually reconcil’d to us, till by faith we lay hold on Jesus. We are saved by his meritorious sufferings; but not unless they be applied and appropriated unto us by faith; namely, when every one of us can particularly say, from an inward sense and persuasion in his heart, and from a secret virtue and change which he feels there, “the Son of God hath loved me, and given himself for me, Christ was born for me, suffer’d and died for me, rose again for me, ascended into heaven, and there intercedeth for me; in a word, all his undertakings were for me and my everlasting benefit.”
Later on, on p. 209-210, he adds this beautiful bit:
…the act of faith whereby we apply the righteousness of Christ to our own souls in particular, cannot but convey an infinite joy to us: for see how it is in secular and worldly matters, if I can cast mine eye on a small parcel of land, and say with truth, that these few acres of ground are mine, that they belong to me as the right owner, this is far more grateful and pleasant to me, than if I should mount a hill, and take a view of a much larger tract of ground; nay, if I should come down, and have the liberty to ride or walk in it, to feed my eye, and almost lose it in surveying its vast extent, but then after all must say, this belongs to my neighbour, not a foot of these fair fields is mine. If it be thus in temporal and worldly things, it is much more in those that are spiritual. If I can only say, there are great and precious promises in the Gospel, there are vast privileges purchased by Christ’s undertakings, sinners may partake of all benefits and blessings by his blood; but if I cannot add, that I have an interest and propriety in them, I have no ground in rejoicing. What comfort is it to a man to be told, that the sun shines, when he is pent up in a dungeon, where he never sees the light, or feels the warmth of the sun? But if I can say, and say it truly and on good grounds, that I have a portion in those undertakings, I am particularly concerned in the death and merits of Christ, I have a share in the promises of the Gospel, I can, and do apply his meritorious righteousness to my soul, I rest on Christ, not only as a perfect Saviour, but as my Saviour; if I can say this, I have reason to rejoice and be exceeding glad.