Guthrie is talking to someone who has professed faith but who doubts the reality of his interest in Christ. He recommends self-examination followed by the saying of this (or similar) and so renewing the covenant you entered with God by baptism/confession previously:

O Lord, I am a lost and fallen creature by nature, and by innumerable actual transgressions, which I do confess particularly before Thee this day: and although, being born within the visible church, I was from the womb in covenant with Thee, and had the same sealed to me in baptism; yet, for a long time, I have lived without God in the world, senseless and ignorant of my obligation by virtue of that covenant. Thou hast at length discovered to me, and impressed upon my heart, my miserable state in myself, and hast made manifest unto my heart the satisfying remedy. Thou hast provided by Christ Jesus, offering the same freely unto me, upon condition that I would accept of the same, and would close with Thee as my God in Christ, warranting and commanding me, upon my utmost peril, to accept of this offer, and to flee unto Christ Jesus; yea, to my apprehension, now Thou hast sovereignly determined my heart, and formed it for Christ Jesus, leading it out after Him in the offers of the gospel, causing me to approach unto the living God, to close so with Him and to acquiesce in His offer, without any known guile. And that I may come up to that establishment of spirit in this matter, which should be to my comfort, and the praise of Thy glorious grace; therefore, I am here this day to put that matter out of question by express words before Thee, according to Thy will. And now I, unworthy as I am, do declare, that I believe that Christ Jesus, who was slain at Jerusalem, was the Son of God, and the Saviour of the world. I do believe that record, that there is life eternal for men in Him, and in Him only. I do this day in my heart approve and acquiesce in that device of saving sinners by Him, and do intrust my soul unto Him. I do accept of reconciliation with God through Him, and do close with Thee as my God in Him. I choose Him in all that He is, and all that may follow Him, and do resign up myself, and what I am, or have, unto Thee; desiring to be divorced from everything hateful unto Thee, and that without exception, or reservation, or anything inconsistent within my knowledge, or any intended reversion. Here I give the hand to Thee, and do take all things about me witnesses, that I, whatever I be, or have hitherto been, do accept of God's offer of peace through Christ; and do make a sure covenant with Thee this day, never to be reversed, hoping that Thou wilt make all things forthcoming, both on Thy part and mine, seriously begging, as I desire to be saved, that my corruptions may be subdued, and my neck brought under Thy sweet yoke in all things, and my heart made cheerfully to acquiesce in whatsoever Thou dost unto me, or with me, in order to these ends. Now, glory be unto Thee, O Father, who devised such a salvation, and gave the Son to accomplish it: Glory be to Christ Jesus, who, at so dear a rate, did purchase the outletting of that love from the Father's bosom, and through whom alone this access is granted, and in whom I am reconciled unto God, and honorably united unto Him, and am no more an enemy or stranger: Glory to the Holy Ghost, who did alarm me when I was destroying myself, and who did not only convince me of my danger, but did also open my eyes to behold the remedy provided in Christ; yea, and did persuade and determine my wicked heart to fall in love with Christ, as the enriching treasure; and this day does teach me how to covenant with God, and how to appropriate to myself all the sure mercies of David, and blessings of Abraham, and to secure to myself the favour and friendship of God for ever. Now, with my soul, heart, head, and whole man, as I can, I do acquiesce in my choice this day, henceforth resolving not to be my own, but Thine; and that the care of whatsoever concerns me shall be on Thee, as my Head and Lord, protesting humbly, that failings on my part (against which I resolve, Thou knowest) shall not make void this covenant; for so hast Thou said, which I intend not to abuse, but so much the more to cleave close unto Thee, and I must have liberty to renew, ratify, and draw extracts of this transaction, as often as shall be needful. Now, I know Thy consent to this bargain stands recorded in Scripture, so that I need no new signification of it; and I, having accepted of Thy offer upon Thine own terms, will henceforth wait for what is good, and for Thy salvation in the end. As Thou art faithful, pardon what is amiss in my way of doing the thing, and accept me in my Lord Jesus Christ, in whom only I desire pardon. And in testimony hereof, I set to my seal that God is true, in declaring Him a competent Saviour.

For myself; the syllogistic reasoning does not work in that I agree that I am a sinner, that Christ died for sinners, that if you have faith then you will be saved. I know that I believe in Christ but I am not assured that my faith is saving faith. Prior to my "conversion experience" I was not in tears over my sin, so I am led to question the reality of that "conversion experience" though I have since then mourned my sin, but then was that a legal mourning over fear from hell or an evangelical mourning our hatred of sin and anger at it knowing that it displeases God? (Rhetorical question for if I am not sure then you will not know).

I read The Inward Experience of Believers by Robert Murray M'Cheyne last noght which helped a little.

'Tis a point I long to know,
Oft it causes anxious thought,
Do I love the Lord or no?
Am I His, or am I not?

__________________
Richard
CofE
UK