The sin of presumption is more a matter of being convinced -- arrogantly -- that one has gained guaranteed admission to heaven and is no longer in need of God's Mercy (as many Protestants are).
OK. Now I see where the error is.
There's not a born again believer who thinks that just because they are sure of their salvation that they don't need God's mercy.
We realize that anything we have at all is a result of God's mercy. Not only our salvation, but the assurance of it as well.
My assurance of salvation is based on the promises of God that HE gave to us.
Taking Him at His word is not presumption, but faith.
I’m saved and secure in that not because I think so, not because I think I deserve it for any reason, but because HE promised that if I believe Him, then just like Abraham, my faith is counted to me as righteousness.
He cannot lie. His word is true and sure. I can have confidence on THAT basis, and none other.