Well, I have been unable to find a clear statement in the Fathers that says, verbatim, that the centrally highlighted "office" of Christ in the New Testament is His Kingship. However, I am unsure that there would be such a quote. Not a tremendous amount really hinges on this point, since obviously Christ is infinitely more than a King, a fact I greatly appreciate. One of my favorite Orthodox writings is the Akathist to the Sweetest Lord Jesus, from which one cannot help but get extremely excited about the multiplicity of His traits...they are indeed incalculable. I can say, however, that Father Hopko says this in perhaps as many words - that is certain. While he is not a church Father, or a Saint, he is clearly speaking from an advanced Orthodox perspective. In preparing this lesson for my church, I remembered that Father Hopko has a series on the names of Christ, and so I listed to it to see what he thought. You can read a wonderful transcript of his teaching on this here: http://www.ancientfa...esus_-_the_king
He does clearly state the centrality of this point in scripture, but he stops just barely short of saying it is "the most important". He almost says it, but stops himself!
This is the Christian worldview. This is the Gospel. This is the understanding. So we could actually say—I think it would not be wrong to say—that perhaps the term “the King” is probably, biblically, the most—I wouldn’t say it’s the most important title for Jesus, because it’s synonymous with “God’s Son” and it’s synonymous with “Lord” and it’s synonymous with “Christ.” God’s Son, the Lord, the Christ, is the King and has to be the King. And then the King has to be the Savior, which is why Jesus is named “Jesus,” which means “Savior.”
He never does finish the sentence..."the most..." In listening and reading to his lesson, you can discern, though, that he is saying it is the most prevalent theme in the Bible. I do not intend to put words in his mouth, but I am confident that is how he would have finished the sentence...if he had. If the teaching of the Kingship of Jesus Christ (well, and the Kingship of God in general) is the central theme of Scripture, that does not in any sense make it the most "important" (as Fr Hopko says above). No trait is more or less important, they cannot be...because God is not "more or less" anything. The question, in the original post, was simply related to the key point, or key theme, of Scripture (or at least the NT)...not which was or was not the most "important". Emphasis doesn't really ever indicate importance, since importance is circumstantial or subjective. It appears that the emphasis is on God/Christ's Kingship in the New Testament, because of the announcement of the Reign of God, or of the Kingdom of God having come was the central message...emphasis on that point in the Bible does not detract from, say, Christ as Savior...since obviously if it were not for Jesus as Savior, we would have a very different future, well, and a very different King.