His saying:

means, do not be of the idolators who split up their religion, i.e., changed it by believing in parts of it and rejecting other parts. Some scholars read this as Faraqu Dinahum, meaning “neglected their religion and left it behind them.”

These are like the Jews, Christians, Zoroastrians, idol worshippers and all the followers of false religions, besides the followers of Islam, as Allah says:

Verily, those who divide their religion and break up into sects, you have no concern in them in the least. Their affair is only with Allah.

The followers of the religions before us had differences of opinions and split into false sects, each group claiming to be following the truth. This Ummah too has split into sects, all of which are misguided apart from one, which is Ahlus-Sunnah Wal-Jama'ah, those who adhere to the Book of Allah and the Sunnah of the Messenger of Allah and what was followed by the first generations, the Companions, their followers, and the Imams of the Muslims of earlier and later times. In his Mustadrak, Al-Hakim recorded that the Messenger of Allah was asked which of the sects was the saved sect and he said:

«What I and my Companions are upon.»