John 6:38-40 does not explain what "seeing and believing" in Christ exactly means. The next episode of the same chapter shows that, for example, those unable to comprehend the Real Presence of Christ in the Eucharist left Him. Christ's promise a few verses down, "except you eat the flesh of the Son of man and drink his blood, you shall not have life in you. He that eateth my flesh and drinketh my blood hath everlasting life: and I will raise him up in the last day" (John 6:54-55) excludes them. But they had been among His disciples initially and up till that moment had had faith in Him.
Likewise, Christ warns specifically that "Not everyone who says to me, 'Lord, Lord' shall enter the kingdom of heaven" (Matt. 7:21). St. Paul advises to "work out your own salvation with fear and trembling" (Phil 2:12).
These verses point to a need for a continuing cycle of confession, repentance, and spiritual growth, -- a need to battle sin with perseverance -- before we can be sure of our salvation. Hope we must. Presume -- cultivate inordinate hope -- is a cardinal sin, related to pride. Despair, incidentally, is another case of inordinate hope, also sinful. Aquinas actually holds it to be more sinful than presumption.
How can we grow in hope and not fall into presumption or despair? We know that the perfect sacrifice of Christ is sufficient to save us all, without exception. We know the commandments of Christ. Christ gave us visible means of grace through the sacraments of Confession and Eucharist. We thus know that having confessed and repented our sins, and having received the Body of the Lord, -- physical actions that cannot be misinterpreted, -- we enter a state of grace which, are we to die the next moment, assures our salvation. This, -- not the presumption of salvaton based on a single declarative act of untested faith, -- is the right hope of the Gospel.
Presumption can be of two kinds. The Pelagian presumption of one's own capacity is the kind we have no dispute about; at issue is the second type of presumption, of salvation regardless of one's actions.
"Assures your salvation" only for that moment, correct?
Because 20 minutes later, you may and probably will sin again, and thus the cycle repeats itself, ad infinitum, until death.
Sin, sacrament, forgiveness, perfection. Sin, sacrament, forgiveness, perfection. Sin, sacrament...
The real trick becomes dying at the correct moment in the cycle.
But Scripture tells us our sins were forgiven on the cross. Christ paid the price for our transgressions, once for all time. To believe that single fact is to be born again by His grace.
And not only so, but we also joy in God through our Lord Jesus Christ, by whom we have now received the atonement. Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned: For as by one man's disobedience many were made sinners, so by the obedience of one shall many be made righteous." -- Romans 5:10-12,19"For if, when we were enemies, we were reconciled to God by the death of his Son, much more, being reconciled, we shall be saved by his life.
Our new birth is assured by Christ's obedience, and not our own, since all our righteousness is as filthy rags. That is the great comfort you are missing -- that our salvation does not depend on our good works, but on the accomplished work of the Holy Ghost within us.
This glorious "presumption" is given by God for the comfort of those who belong to Him. I presume it joyfully, gratefully.
For God, who commanded the light to shine out of darkness, hath shined in our hearts, to give the light of the knowledge of the glory of God in the face of Jesus Christ. But we have this treasure in earthen vessels, that the excellency of the power may be of God, and not of us. We are troubled on every side, yet not distressed; we are perplexed, but not in despair; Persecuted, but not forsaken; cast down, but not destroyed; Always bearing about in the body the dying of the Lord Jesus, that the life also of Jesus might be made manifest in our body. For we which live are alway delivered unto death for Jesus' sake, that the life also of Jesus might be made manifest in our mortal flesh. So then death worketh in us, but life in you. We having the same spirit of faith, according as it is written, I believed, and therefore have I spoken; we also believe, and therefore speak; Knowing that he which raised up the Lord Jesus shall raise up us also by Jesus, and shall present us with you. For all things are for your sakes, that the abundant grace might through the thanksgiving of many redound to the glory of God. For which cause we faint not; but though our outward man perish, yet the inward man is renewed day by day. For our light affliction, which is but for a moment, worketh for us a far more exceeding and eternal weight of glory; While we look not at the things which are seen, but at the things which are not seen: for the things which are seen are temporal; but the things which are not seen are eternal." -- 2 Corinthians 4:5-18."For we preach not ourselves, but Christ Jesus the Lord; and ourselves your servants for Jesus' sake.
"I am crucified with Christ: neverthless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me.
I do not frustrate the grace of God: for if righteousness come by the law, then Christ is dead in vain." -- Galatians 2:20-21
Our faith is the result of His sacrifice, determined by God from before the foundation of the world as the means to bring His sheep home. Our faith is not an incomplete faith, requiring a perpetual restoration. It is an abiding, constant and eternal faith, finished in His perfect work on the cross.
Men would have it otherwise in order to better keep us uncertain. Same as it ever was.
But as Paul instructs us, "Do not frustrate the grace of God" by doubting the "presumption" of His accomplished redemption.