In this event, asking for permission, in the case of a person who believes in Allah and the Last Day, expresses an instability of faith. This is because he likens the pleasures of this worldly life with the rewards of the Hereafter. Abstaining from joining the Messenger of Allah (saws) reflects a prioritization of worldly pursuits over the Hereafter, and choosing this temporary life over the eternal bliss of the Hereafter. Therefore, Allah (swt) said:

(لَا يَسْتَأْذِنُكَ الَّذِينَ يُؤْمِنُونَ بِاللَّهِ وَالْيَوْمِ الْآخِرِ) (Those who believe in Allah and the Last Day would not ask permission of you…): One may ask: Why does Allah (swt) mention here faith in Allah and the Last Day? It is because the pinnacle of faith is believing in Allah (swt) and the culmination of the arc is faith in the Last Day. It is true that faith encompasses belief in Allah, His angels, His books, His messengers, the Last Day, and divine decree, but the Last Day is foundational. When you believe in Allah (swt) you must also believe that there is a day on which you will be held accountable. You must acknowledge that there is a Day when you will stand before Allah (swt) the One in whom you have believed, and who has informed you about the existence of Paradise and Hellfire.