Surah Al Hijr Tafseer

As We have sent down on the Muqtasimin (conspiring confederates),

المُقْتَسِمِينَ

(the Muqtasimin) refers to those who had made a pact to oppose, deny, and insult the Prophets.

Similarly, Allah tells us about the people of Salih:

قَالُواْ تَقَاسَمُواْ بِاللَّهِ لَنُبَيِّتَنَّهُ وَأَهْلَهُ

They said, "Swear to one another (Taqasamu) by Allah that we shall make a secret night attack on him and his household." (27:49)

i.e., they plotted to kill him at night.

Mujahid said;

"Taqasamu means they swore an oath."

وَأَقْسَمُواْ بِاللَّهِ جَهْدَ أَيْمَـنِهِمْ لَا يَبْعَثُ اللَّهُ مَن يَمُوتُ

And they swear by Allah with their strongest oaths, that Allah will not raise up one who dies. (16:38)

أَوَلَمْ تَكُونُواْ أَقْسَمْتُمْ مِّن قَبْلُ

(It will be said):"Did you not before swear that you would not leave (the world for the Hereafter)! (14:44)

أَهَـوُلاءِ الَّذِينَ أَقْسَمْتُمْ لَا يَنَالُهُمُ اللَّهُ بِرَحْمَةٍ

Are they those, of whom you swore that Allah would never show them mercy. (7:49)

It is as if they took an oath for every single thing that they denied in this world, so they are called the Muqtasimin.

Who have made the Qur'an into parts.

meaning, they have split up the Books that were revealed to them, believing in parts of them and rejecting parts of them.

الَّذِينَ جَعَلُوا الْقُرْانَ عِضِينَ