Imam al-Baghawi (d. 516H) in his book “Sharh as-Sunnah” while commenting on the hadith about the Khawarij:
وَقَوْلُهُ: «يَمْرُقُونَ مِنَ الدِّينِ» أَيْ: يَخْرُجُونَ مِنَ الدِّينِ، أَيْ مِنَ طَاعَةِ الأَئِمَّةِ، وَالدِّينُ: الطَّاعَةُ، وَهَذَا نَعْتُ الْخَوَارِجِ الَّذِينَ لَا يَدِينُونَ لِلأَئِمَّةِ، وَيَسْتَعْرِضُونَ النَّاسَ بِالسَّيْفِ،
Here he explains the trait mentioned in the hadith about the Khawarij that they read the Qur’an but “do not go beyond their throats” that it means “Neither accepted nor raised are their good deeds“.
In other words, even if the Khawarij read the Qur’an, pray tahajjud, fast every week, fight their self declared “jihad”, or assume about themselves that “at least they have good intentions and calling to Tawhid and Shariah”, it does not matter in light of the religion that Allah has sent down and their deeds are deemed worthless.
And with regards to the description that the Khawarij “pass through the religion”, Imam al-Baghawi explains that “this attribute of the Khawarij is because they denounce the Imams and view people with the sword”.