Saying something is "built on" the testimony of the Apostles is not the same as saying that it is what the Apostles themselves taught. It seems to me that the creeds add extra-Biblical concepts to the teachings of Jesus and the Apostles.
But I do not want to argue the point with you. I asked for the definition of Christian. Clearly, you think that the ecumenical creeds are important in part of any such definition.
Any "protestant" who could not affirm the statements in the ancient creeds cited should have their Christianity questioned. There is much bad teaching among the "no creed but Christ" crowd, including poor constructs on the Trinity that lead to a form of modalism. The very purpose of the creeds is to sort out the bad theology among the faithful. And to help keep cults and heresies (wolves) away from the flock.
I gather that you do not approve of the "non-creedal" churches. But are they Christians? If not, what would you call them?
Your statement is an extra-biblical concept of what Jesus and the apostles taught. It is your creed. It may be accurate or it may not. It stands or falls on your own ability to accurately determine the teaching of Christ and His apostles.
At least the historic creeds have the advantage of wisdom that comes from a number of counselors (Prov. 11:14).
I gather that you do not approve of the "non-creedal" churches. But are they Christians? If not, what would you call them?
Every Christian has a creed. Every church has a creed. E.g., "No creed but Christ" is a creed.
I would see churches that attempt to deny they have a creed or pooh-pooh the historic creeds as merely illogical and aberrations within historic Christianity.
All heretics start with the Bible. The only way I can know one of these folks or groups espouses the true Christian faith is to show them the historic creeds and ask them whether they agree or disagree.