Overall meaning : Allah commands the People of the Book—both Jews and Christians—to believe the Qur’an revealed to Muhammad, may Allah bless him and grant him peace, confirming their own scriptures before He obliterates their faces and turn them backwards, or excludes them from His mercy and inflicts a severe punishment on them, as he did with the Sabbath-breakers who resorted to trickery to fish on Saturday after they were forbidden to do so, and so He transformed them into apes. What Allah ordains is always accomplished, and His command is bound to be executed.
Then He declares that He will never forgive those who worship others besides Him and that He will pardon whatever is short of that, be they minor or major sins, to whomever He wills of His servants, stating that Whoever associates partners with Him has indeed devised a tremendous sin.