Speaking from a historical and literary context, the OP has inadvertently hit upon one of the great tensions of the old and new testament with this quote.
Messiah was composed in 1741 during the reign of George II and takes this text from the King James version of the bible, published in 1611.
The tension I speak of is the desire for a political messiah vs. the apolitical messiah of personal growth. For many, if we take the gospels as accurate, Jesus was not the messiah that was wanted. They wanted a messiah that centered the Israelites as a dominant political force. A new David or Moses was what was being asked, which Jesus was not about to be. I point this out because this tension, the need for divine right to rule never really goes away. Certainly not by Handel’s time. Honestly this argument, that Jesus was apolitical and our politics should be a-religious is quite modern. The separation of church and state in our founding documents was passionately argued for and against.
No one was having an argument over whether Kings should be appointed by God or not, they were arguing whose version of God should be appointing kings, and hence the birth of the Church of England.
AFAIK, those who supported King George II and those who opposed were often divided among religious labels.
What’s the point? That a writer in these times might not have been keen on taking up the fight for the "true" Jesus/Messiah and their true place in political power anymore than we are willing to take up that fight now. Alluding that the then current political king of England as standing on the shoulders of Jesus seems quite normal for the time in which it was written.
PS - I adhere to none of these religious beliefs and I only comment upon the undercurrents which might have resulted in this musical work.