Al-Ma'idah: 17
"They have certainly disbelieved who say..."
"That Allah is the Messiah"
The meaning is a definitive statement that the reality of Allah is the Messiah, and nothing else. It is said that there were people among the Christians who said this explicitly. It is also said that they did not state it explicitly, but their doctrine leads to it, as they believed he creates, gives life, causes death, and manages the affairs of the world.
"Say, 'Then who could prevent Allah at all...'"
Meaning: Who could prevent anything from His power and His will?
"...if He intended to destroy..."
...those whom they called gods, namely the Messiah and his mother. This is evidence that the Messiah is a created servant like all other servants.
He intended by linking "whoever is on the earth" to "the Messiah and his mother" [to show] that they are of the same kind as them, and there is no difference between them and the rest of humanity in terms of being human.
"He creates what He wills"
Meaning: He creates from a male and a female, and He creates from a female without a male—as He created 'Isa (Jesus)—and He creates without a male or a female—as He created Adam.
Or, He creates what He wills, such as the creation of the bird at the hands of 'Isa as a miracle for him, and the reviving of the dead, and the healing of the blind and the leper, and other such things. Therefore, it must be attributed to Him [Allah], and not attributed to the human through whose hands it was performed.