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.
But a typical Protestant doesn't believe Him. He believes some parts but not others. For example, you don't believe that the Holy Eucharist saves, despite His words. You have that laughable doctrine that salvation is by faith alone, even though the Holy Scripture says the opposite in the Second Chapter of James. You, collectively, are cafeteria Christians.