One of the most misunderstood parts of Catholicism is the notion of the sin of presumption. Protestants who are big on “assurance of salvation” often claim that Catholics can’t know that they’re saved, because to do so would be the “sin of presumption,” as understood by the Church. Then, they trot out Bible verses like 1 John 5:13, which says the following: “These things I have written to you who believe in the name of the Son of God, that you may know that you have eternal life, and that you may continue to believe in the name of the Son of God.”
And if they understood the sin of presumption, this would be a good argument (and in fact, it is a good argument to convince misinformed Catholics to turn their back on authentic Catholic teaching). But the sin of presumption isn’t saying “I’m saved.” The sin of presumption is saying, “No matter what the future holds, I’m saved.” If you believe in Once Saved, Always Saved (OSAS), this distinction probably makes no sense – so it’s reasonable that even well-intentioned Protestants would misunderstand this notion (particularly since lots of Catholics seem misinformed or ignorant of it).
If we truly believe in Jesus Christ, and are in a right relationship to Him, we can say with 100% assurance, “I’m saved.” Not only is there no sin there, but it’s foundational to effective evangelization: someone who isn’t saved can rarely be an effective servant of the Gospel. But since Catholics reject OSAS, we think that just because you’re in a right relationship with God now, and just because God is faithful, you can still be unfaithful. Saying, “no matter what the future holds, I’m guaranteed a spot in Heaven,” is wrong not because it puts too much Faith in God, but because it puts too much faith in ourselves.
In fact, we see the evidence of this in 1 John 5:13 itself, given how John explains his purpose: “These things I have written to you who believe in the name of the Son of God, that you may know that you have eternal life, and that you may continue to believe in the name of the Son of God.” So John says that those who believe in the name of Jesus have eternal life – now. But he’s still writing to them so that they will continue to believe. This only makes sense if it’s possible that they won’t continue to believe. And note that his scope is narrow, he’s writing “to you who believe in the name of the Son of God,” that is, those currently saved.
So, in short, the sin of presumption is not saying “you who believe in the name of the Son of God … may know that you have eternal life.” It’s saying “you who believe in the name of the Son of God … are assured that you will always continue to believe in the name of the Son of God, no matter what.”