Let me clarify a little for you. In chapter 1 of Revelation John mentions that he was transferred into the 'Lord's day.' This would be 1914 and onward. So I believe that yes the letter to the 7 congregations are for the congregations in the Lord's day since 1914. But as I said in my answer previously Jesus was king of kings even while on earth. So even if you don't want to believe the letters are for the congregations since 1914, that is ok. Jesus being exalted to heaven at his ressurection is still of course the rightful king and by being at the right hand of God he could certainly be called ruler of the king's of the earth. Even when on earth he could be called king of kings because he was the chosen king to rule over the earth.
But then I later expounded on my answer when I wrote this:
Notice Revelation 12 which was to occur in the Lord's day after Jesus statement at Matt 28:18. Notice verse 15 And the seventh angel blew his trumpet. And loud voices occurred in heaven, saying: “The kingdom of the world did become the kingdom of our Lord and of his Christ, and he will rule as king forever and ever.” Chirst was to receive authority in a special way in the future. But is this really so? After Satan is cast out of heaven Revelation 12:10 tells us: And I heard a loud voice in heaven say: “Now have come to pass the salvation and the power and the kingdom of our God and the authority of his Christ, because the accuser of our brothers has been hurled down, who accuses them day and night before our God! So even though Jesus was given authority after his ressurection he did not exercise all his authority until the proper time appointed by his Father. He waited until he received his kingship over God's Messianic Kingdom. And even still he has not exercise all his authority because he has not destroyed all the other kingdoms. (Daniel 2:44) Daniel 7 also shows us that it is during the last wild beast of the earth, the time of the small horn that grows big(Rev 17:10 tells us there would be seven. "And there are seven kings: five have fallen, one is (Rome), the other has not yet arrived (Anglo-american)) that Jesus gains access to the throne of God. Well just notice:
13 “I kept on beholding in the visions of the night, and, see there! with the clouds of the heavens someone like a son of man happened to be coming; and to the Ancient of Days he gained access, and they brought him up close even before that One. 14 And to him there were given rulership and dignity and kingdom, that the peoples, national groups and languages should all serve even him. His rulership is an indefinitely lasting rulership that will not pass away, and his kingdom one that will not be brought to ruin.
For further proof of this lets look at an illustration that Jesus gave: Luke 19:11 While they were listening to these things he spoke in addition an illustration, because he was near Jerusalem and they were imagining that the kingdom of God was going to display itself instantly. 12 Therefore he said: “A certain man of noble birth traveled to a distant land to secure kingly power for himself and to return. 13 Calling ten slaves of his he gave them ten mi´nas and told them, ‘Do business till I come.’ 14 But his citizens hated him and sent out a body of ambassadors after him, to say, ‘We do not want this [man] to become king over us.’
15 “Eventually when he got back after having secured the kingly power, he commanded to be called to him these slaves to whom he had given the silver money, in order to ascertain what they had gained by business activity.
Well of course you know the story. He rewarded those who had obeyed his commands. But as for the enemies of his look what happened.
27 Moreover, these enemies of mine that did not want me to become king over them BRING here and slaughter them before me.’”
This proves: Jesus would not secure kingly power until sometime after his accension to heaven. So either the Bible contradicts itself or Jesus was to receive kingship in a special way and authority in a special way after a long period of time in the final part of the days. By being appointed to this special kingship as ruler of God's kingdom that was to destroy all other kingdoms no longer would his authority have an affect only on his congregation but it would affect all of mankind. Jesus even though having all authority waited until this special appointment before he exercised that authority to its greatest extent. And still he has not exercised all of his authority. He will do that at Armageddon. I realize that some like AlanF will take the option that the Bible contradicts itself and its prophecy have failed but for those who still believe the Bible is inspired of God, we can see that the Bible all harmonizes and that the explanation is both scriptural and reasonable.