What does "that concerning which they differed" refer to in the āyah?

Imām Ibn al-Jawzī (d. 597/1201) summarized the scholars' opinions in his book "Zād al-Masīr" as follows:

1. It was regarding the day of Friday. The Jews made their sacred day Saturday, and the Christians made it Sunday. Al-Bukhārī and Muslim narrated in their Sahihs from a hadith of Abū Hurayrah that the Prophet (ﷺ) said: "We are the last of the nations, but the first on the Day of Resurrection, even though they were given the Scripture before us, and we were given it after them. This is their day which was enjoined upon them, but they differed concerning it, so Allah guided us to it. Thus, today (Friday) is for us, tomorrow (Saturday) is for the Jews, and the day after (Sunday) is for the Christians."
2. It was about the prayer. Some pray towards the east, and others towards the west.
3. It was about Abraham. The Jews claimed he was Jewish, while the Christians claimed he was Christian.
4. It was about Jesus. The Jews regarded him as illegitimate, while the Christians considered him divine.
5. It was regarding the scriptures. They believed in some and disbelieved in others.
6. It was about religion itself, which is the most correct opinion because all the other interpretations are subsumed within it.