This is a false premise from which to begin. It assumes the faith by which we are bound to Christ comes from imperfect us. It does not.
I agree. The ability to know something is true that you can not know is true through any human faculty--in this case that the gospel is true--is purely a gracious gift of God. Nobody can manufacture the faith to know if something is true that they have no way of knowing on their own if it's true or not. Faith is not deciding something is true--aka, a 'blind' faith. Faith is
knowing something is true.
In the case of the gospel this supernatural ability to know the gospel is true comes through the Word of the gospel itself. It's the Holy Spirit that delivers the message of the gospel to the heart accompanied by the ability, through that same Spirit, to know it's for real. From there men and women choose to accept that truth or reject it. Those who reject the truth that they now know without a doubt to be the undeniable truth are calling the Holy Spirit who showed them that truth a liar (1 John 5:10). IOW, they are blaspheming the Holy Spirit--calling that which is utterly good and holy evil and attributing it's work to that of the Devil (lying).
So, the question is, once a person receives the truth about the gospel through the supernatural gracious gift of faith, then believes that truth, accepting it and putting their trust in it, can a person then change their mind about it later and lose the declaration of justification their trust in that truth secured for them?
Many, many people at one time or another in their lives are shown that the gospel is true through the gracious gift of faith convicting them of the truths connected with the gospel, but few actually do the 'work' of believing--placing their trust in the truth delivered to them through faith (the supernatural ability to realize something is true) via the Holy Spirit.
"14 For many are called (are given the faith to know the gospel is true), but few are chosen (actually place their trust in the truth God has shown them to be true).”" (Matthew 22:14 NASB)
It's the difference between just having
faith--knowing the facts are true about sin, righteousness, and the judgment to come--many people will say that, and
believing and actually putting your trust in those facts and being justified and, therefore, saved on the Day of Wrath. Jesus really does call 'believing' a 'labor'...something you do (John 6:28-29).
That faith comes from Christ. Therefore, its sufficiency is beyond question, and one justified by faith can never be shaken from it, because that faith is not man-generated, but God-given.
True beyond any doubt whatsoever. But what if I do what John says and call the Holy Spirit who showed me the truth in my heart, beyond a shadow of a doubt, a liar by rejecting the faith to know that the gospel is absolutely true? Can a justified person, one who has gone beyond simple acknowledgement of truth and accepted that truth, do that, or are they irretrievably incapable of now being able to reject the voice of faith and stop believing (trusting) in the blood that they originally accepted as able to save them? Can a person stop the 'work' of believing and reject the faith--the power and ability to know the gospel is true--thus removing themselves from the sufficiency of Christ's blood? That, in my mind, is the real question.