It is not the "sin of presumption" for the Christian to believe what has been revealed in Scripture.
22At that time the Feast of the Dedication took place at Jerusalem; 23it was winter, and Jesus was walking in the temple in the portico of Solomon. 24The Jews then gathered around Him, and were saying to Him, “How long will You keep us in suspense? If You are the Christ, tell us plainly.” 25Jesus answered them, “I told you, and you do not believe; the works that I do in My Father’s name, these testify of Me. 26“But you do not believe because you are not of My sheep.
27“My sheep hear My voice, and I know them, and they follow Me; 28and I give eternal life to them, and they will never perish; and no one will snatch them out of My hand.
29“My Father, who has given them to Me, is greater than all; and no one is able to snatch them out of the Father’s hand. 30“I and the Father are one.”
John 10:22-30 NASB
The Greek behind v28 where Jesus says they will never perish has the strongest negation in Greek possible. John has recorded this statement by Jesus so we'd have this assurance of salvation.
John also noted this toward the end of his Gospel.
30Therefore many other signs Jesus also performed in the presence of the disciples, which are not written in this book; 31but these have been written so that you may believe that Jesus is the Christ, the Son of God; and that believing you may have life in His name. John 20:30-31 NASB
In 1 John he wrote the following:
13These things I have written to you who believe in the name of the Son of God, so that you may know that you have eternal life. 1 John 5:13 NASB
John is recording to the believer in Christ they can have assurance of their salvation as that is what he heard Christ say.
Well, I hope one’s faith in Jesus is really true and one does not die in mortal sin. Even the Pharisees expressed faith in God, and you have read that Jesus told us not to be like them. Jesus will make a decision on justice at the date of our death.
Jesus told us to be humble like children, not like the Pharisees.
So if one does not believe in the Real Presence of Jesus Christ in the Eucharist, then does one have true faith in Jesus?
Although, you *thought* you were saved, and so did everyone around you. In some cases, these "never saved in the first place" folks end up in ministerial positions in Protestant churches! Clearly you have no infallible test for "really saved in the first place", so pretending that can infallibly know that you won't throw away something you aren't really sure you even have is just stuff and nonsense.