What is the oldest denomination?

The very early days of Christianity apparently saw considerable diversity within the Christian community, as evidenced by the comments of authors such as Paul and Matthew. It may be that the earliest Christian sect was called the Ebionites, however, the Ebionites soon faded from history.

Most early Christian sects are generally known today as Catholic-Orthodox Christianity and Gnosticism. We can not really say whether Gnosticism preceded the Catholic-Orthodox Christian faith, or the other way around, although modern churches do insist that Gnosticism arose after Catholic-Orthodox Christianity. However, Gnosticism was never a centalised faith, and never had a strong hierarchy, so was at a disadvantage against the Catholic-Orthodox Christian faith. When Emperor Constantine gave state patronage to Catholic-Orthodox Christianity and authorised it to take over the assets of the Gnostic Churches, the Gnostic Churches were ultimately doomed.

The Great Schism of 1054 CE saw Catholic-Orthodox Christianity finally split into the Roman Catholic and the Orthodox branches, which are equally old.