Al Mumtahinah V.1-13
Perhaps Allāh will put, between you and those to whom you have been enemies among them, affection. And Allāh is competent, and Allāh is Forgiving and Merciful. [Al Mumtahinah:7]
This surah is regarding the women who would migrate to Madinah and they left their families in Makkah. They would have to be tested by the Prophet (saw) to see whether they were actual believers and could stay or not. To me this is the greatest lesson; in verse seven Allah mentions that those who are your enemies could become your beloved. This shows the state of our heart is always changing and no state is permanent; because of this we should prepare ourselves to be tested based on our proclamation of Islam.