Look at the text again! The promise is "of faith" but if faith were "of works" or "of man" as you demand, he could not go on to say it is "by grace" because grace does not originate with man or of man's works. It is "of faith" because faith originates with God's grace and that is why it is "by grace" that the promise is obtained.
So it is saying the very exact opposite of what you are saying. Furthermore, because it is "OF FAITH" it is "SURE" = CERTAIN to all the seed. This also contradicts your definition of faith since your kind faith has NO CERTAINTY so anything that is "OF" your kind of "FAITH" cannot be "sure"!
There are only two ways to receive salvation from God, either you work for it, or you receive it by faith.
If you work for salvation, then salvation is merited or earned and cannot be by grace.
Therefore the opposite must be true, salvation must be received by faith to be by grace. This is what Paul is saying, no matter how you try to twist scripture.
Paul didn't say, "Therefore it is of grace, that it might be by faith" as you try to falsely teach. No, God said it is of faith, that it might be by grace.
Once again, Calvinism teaches the EXACT OPPOSITE of scripture.
As for the promise being SURE, or course it is, I agree 100%. But we must believe that by faith, and faith is believing what is not seen.
It would be like loaning a stranger your car. You believe him by faith that he will return it. While he is gone with your car, you are in a state of faith, believing he will return it as promised.
The moment he returns the car and gives you the keys, you can no longer believe him by faith to return you car, now you know for an absolute certainty that he returned the car.
Faith is depending on someone's promise. You do not depend on something you have in your hand at the moment. Of course, all of God's promises are SURE, because God cannot lie. Nevertheless, we must wait by faith for the fulfillment of those promises.
And anybody who says they have never doubted ever is completely full of bunk, or else they are crazy. Even John the Baptist doubted Jesus, and he was a prophet who was shown who Jesus was by the Holy Spirit.
Mat 11:2 Now when John had heard in the prison the works of Christ, he sent two of his disciples,
3 And said unto him,
Art thou he that should come, or do we look for another?
When John the Baptist was thrown in prison, he became discouraged and began to doubt. He sent messengers to ask Jesus if he was the promised Christ, or should they look for another.
This was AFTER the Holy Spirit had revealed to John that the Holy Spirit would lite on the Christ.
Jhn 1:32 And John bare record, saying, I saw the Spirit descending from heaven like a dove, and it abode upon him.
33 And I knew him not:
but he that sent me to baptize with water, the same said unto me, Upon whom thou shalt see the Spirit descending, and remaining on him, the same is he which baptizeth with the Holy Ghost.
34 And I saw, and bare record that this is the Son of God.
The Holy Spirit himself had told John the Baptist that he would see the Spirit descending and remaining on the Son of God, and yet John doubted later.
Of course, you are so full of yourself that you probably believe you are better than John the Baptist.