I don't see the opening proposition as a negative thing though, it is pro-active, in that it it affirms what we believe as opposed to what we don't believe. Unfortunately, we cannot ignore the context in which the creed was written, which was the direct response to the Arian heresy and in an environment Greek polytheism. I see the creed as a benchmark of faith more than an evangelical tool. The Gospels/good news/glad tidings are our evangelical tools, with the epistles as are tools of greater understanding and last of all creedal statements to "sharpen up the edges."
I don't think you're wrong in taking the spiritual meaning out of the passage, for I believe the creed to be God inspired, and like all of God's words, is fluid and capable of revealing different facets of truth. I think it can have a more plain meaning as well, but I like your take better, it could be translated: "We have faith in one God". The opener then can be understood in two ways, spiritual and legalistic - where our faith abides, and a defining aspect of the nature of God.