This might be a first draft of an answer, in which I will explain your quote to the best of my knowledge:
> The Messiah, son of Mary, was not but a messenger; [other] messengers have passed on before him.

This shows that 'Isa () was one of Allah's messengers and he had been given the same orders and privileges as any other messenger before or after him
> And his mother was a supporter of truth. They both used to eat food. 

This part shows that both' Isa and his mother Mary () were humans, as both act like humans Allah used the example of eating as we are told in the story of Ibrahim () that angels don't eat.
> Look how We make clear to them the signs; then look how they are deluded.

So Allah gave clear signs of them being humans while others claimed they are the son of God or the mother of God (or the mother of the son of God). So these people who made such wrong claims are deluded. 

The verse clearly is a statement against polytheism and a typical call for monotheism as Islam and the quran are teaching.