Your answer, “first-century Christian,” is certainly accurate since we trace our origin to the Church that Jesus founded in the early first century. This means we worship on Saturdays and observe the biblical holidays as Jesus and the disciples did.
Because we have no association with any other religious organization or any denomination, the term “nondenominational Christian” could also be an answer to give to your friends. Your friends are probably most interested in knowing whether or not you belong to a Christian church, and the term “nondenominational” tells them that your church is not associated with one of the mainstream, large, well-known church organizations.
For more information about the United Church of God, read our free booklet This Is the United Church of God.