This question already has an answer here:
In a previous question about the English of the KJV a link was helpfully supplied and I read the following
The vocative case is used when directly addressing a person with a noun identifying the person instead of with the second person pronoun “you.” An example is in Matthew 6:9 which says, “Our Father, which art in heaven.” Today we are less inclined to say “Our Father, who ARE in heaven.” It seems more natural to say “Our Father, who IS in heaven.” The peculiarity of the KJV is based on the faithful translation of the vocative case. This is not an archaism but a faithful translation of the Greek which has the vocative case.
Like many things, I learned the words as a child and have accepted them all my life without intelligently understanding them.
I am still struggling to understand why the verb is plural and where 'art' comes from.