It's not presumption when God says it's so and we simply choose to believe him - take Him as His word. God is not a liar. If He says we're sealed until the day of redemption, then we're sealed until the day of redemption. Believing what God says is simply called *faith*.
Ephesians 1:13-14 In him you also, when you heard the word of truth, the gospel of your salvation, and believed in him, were sealed with the promised Holy Spirit, 14who is the guarantee of our inheritance until we acquire possession of it, to the praise of his glory.
Ephesians 4:30 And do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption.
You're presuming a great deal. You're presuming you know precisely what you must choose to believe and that you truly believe it and that you will always believe it. You are presuming the future, presuming your choices, and His mercy no matter what either the future or your choices are.
Are you sure you're not a Calvinist?