This is usually a question asked of Holy Scriptures. Self-attesting means that the Scriptures have the power of evidence within it, needing no outside proof of their inspiration. To read the Scriptures is know the “ring of truth,” as JB Phillips wrote.
It seems to me that the Apostles Creed has this same power. To confess it is to be immersed in the truth of it. It stiffens the spine, makes the heart hope, orders the chaos around us, shines the light in the darkening world, imparts energy to the tired and weakened.
Whence comes this power? The AC is a forefronting of the biblical kerygma itself, the Gospel proclamation. It is Christianity. It unites the diverse church and brings harmony out of its disorder, simply stated as it is. It is catechesis – teaching us even while explaining us, as it stands even without comment. It is a canon shot fired at the walls of hell.
Once we begin trying to exposit it in some fashion, it begins to lose its power, for some use the AC to prove their own brand of Christianity. But this is not what it does. It raises Christianity above all brands.
To not confess what is in the AC is to be sub-Christian. Leave anything out and we end up confessing what is less than Christianity, another religions, as it were. Add to it and we begin to fog the looking glass and Christianity becomes the province of sects and heretics.
While I will not speak of a Creed as Holy Scripture, I do wonder whether or not the light that Scripture has shined upon the AC imparts a shadow of its own virtue of self-attestation.