I would ask you, what sin could you commit in the future, after professing belief in Jesus as your needed Savior, that would take God by surprise? When God births your human spirit from Above, His life is put into your human spirit and there is no "sin" which you can thereafter commit which would surprise the One Who knows the end from the beginning. You cannot earn Salvation and you cannot buy it on the installment plan of catholic troddery. The twisted reasoning of carts before horses is familiar. The Bible records ol' Nicodemus doing the same thing in John 3.
Do you have any more questions?