Christianity has never been one thing, and its divisions are old enough to be part of its structure rather than accidents within it.
The first came out of the christological councils. Churches that could not accept the formula agreed at Chalcedon in 451, the Coptic, Syriac, Armenian and Ethiopian traditions among them, have existed separately ever since, which makes them the oldest continuous division in the religion and considerably older than the ones Western histories usually lead with.
The second separated the Greek East from the Latin West. It is conventionally dated to 1054, when legates of the Roman see and the patriarch of Constantinople excommunicated one another, but that is a marker rather than a cause: the two halves had been drifting for centuries over the authority of the bishop of Rome, the wording of the creed, the language of worship, and the political fact of an empire that no longer held both. The sack of Constantinople by a Western crusade in 1204 did more lasting damage than the excommunications.
The third broke the Latin West itself. From 1517 the Reformation set scripture against the authority of the papacy and produced not one alternative but many, Lutheran, Reformed, Anglican, Anabaptist, and those in turn produced more. Most of the several hundred denominations counted today descend from that fracture.
A fourth is still in progress and rarely gets a number. Pentecostal and charismatic Christianity, essentially a twentieth-century phenomenon, now claims a quarter of all Christians and cuts across the older divisions rather than adding another branch to them.