There is your error. They only understand that we are still in this carnal flesh and will fail at times. The difference between Catholics and Protestants is that Protestants stand secure in the promises of Christ.
We understand the difference between wholeheartedly believing in and trusting in God, and self-declaration of one's own salvation.
See, I'm not getting that impression at all. In fact, it sounds like "you" (the we) mistake assurance and trust in the promises of God as "self-declaration" of ones own salvation. Not a one of "us" believes we merit this salvation or deserves it in ANY way. No, it is falling upon the mercy seat of God's grace and taking him at his word that he WILL save us if we receive Christ and trust in his sacrifice for our sins. The only way we can be sure of our salvation is because God DID say we can. In John's epistle I John, he says in chapter 5 verse 13, "These 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." So, it is NOT presumption nor pride in our merit that we say we know we are saved, but simply believing God. He WILL do as he says he will do.