I did not know "presumption" was a "mortal sin." We're told to be modest in our demeanor, but certainly not modest in our faith. On the contrary, we are told to be confident and bold and preach His word to all nations and races on earth so that the elect might hear it and be gathered to Him.
The Pharisee did not lose his salvation; he never had it in the first place.
Does it ever occur to you that men have a vested interest in keeping you unsure of your salvation? I prefer to follow Scripture which is filled with God's assurance to the faithful. We persevere because of His strength, and not our own. God keeps up from falling away because Christ is working within us, and none will be lost of those who belong to His Son.
Either God decreed His elect from before the foundation of the world, or He didn't.
And this is the Father's will which hath sent me, that of all which he hath given me I should lose nothing, but should raise it up again at the last day. And this is the will of him that sent me, that every one which seeth the Son, and believeth on him, may have everlasting life: and I will raise him up at the last day." -- John 6:38-40"For I came down from heaven, not to do mine own will, but the will of him that sent me.
"Always in every prayer of mine for you all making request with joy, For your fellowship in the gospel from the first day until now; Being confident of this very thing, that he which hath begun a good work in you will perform it until the day of Jesus Christ" -- Philippians 1:4-6
And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand. My Father, which gave them me, is greater than all; and no man is able to pluck them out of my Father's hand." -- John 10:27-29"My sheep hear my voice, and I know them, and they follow me:
"Wherefore, my beloved, as ye have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling.
For it is God which worketh in you both to will and to do of his good pleasure." -- Philippians 2:12-13
"He shall choose our inheritance for us" -- Psalm 47:4
"Blessed be the God and Father of our Lord Jesus Christ, which according to his abundant mercy hath begotten us again unto a lively hope by the resurrection of Jesus Christ from the dead,
To an inheritance incorruptible, and undefiled, and that fadeth not away, reserved in heaven for you,
Who are kept by the power of God through faith unto salvation ready to be revealed in the last time." -- 1 Peter 1:3-5
"For non-reformed theologies...at the end of the day, the security of the believer finally rests with the believer. For those in the opposite camp [Reformed], the security of the believer finally rests with God -- and that, I suggest, rightly taught, draws the believer back to God himself, to trust in God, to a renewed faith that is of a piece with trusting him in the first place." -- D.A. Carson
"When God calls a sinner, He does not repent of it. God does not, as many friends do, love one day and hate another; or as princes, who make their subjects favorites and afterwards throw them into prison. This is the blessedness of a saint; his condition admits of no altercation. God's call is founded upon His decree, and His decree is immutable. Acts of grace cannot be reversed. God blots out His people's sins, but not their names." -- THOMAS WATSON
"If our religion be of our own getting or making, it will perish; and the sooner it goes, the better; but if our religion is a matter of God's giving, we know that He shall never take back what He gives, and that, if He has commenced to work in us by His grace, He will never leave it unfinished." -- C.H. Spurgeon
And from the following link comes this clear call to patient security, founded on Christ's sacrifice alone:
Hence comes an extraordinary peace and repose to our consciences. For having disclosed to the Lord the necessity that was pressing upon us, we even rest fully in the thought hat none of our ills is hid from him who, we are convinced has both the will and the power to take the best care of us.{58} When we pray stripped of self-confidence but with confidence in the Lord, we are assured to succeed in that for which we pray. "... We enjoin believers to be convinced with firm assurance of mind that God is favorable and benevolent to them...". {59} We receive this singular fruit of God's promises when we frame our prayers without hesitation or trepidation; but, relying upon the word of him whose majesty would otherwise terrify us, we dare call upon him as Father, while he deigns to suggest this sweetest of names to us. {60}""The result is incredible boldness in Christian living. The Spirit of adoption seals the testimony of the Gospel to us and "raises up our spirits to dare show forth to God their desires." {57}
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.