This O-antiphon explicitly looks to the universality of Jesus. Thus, King of the nations is a title which brings up the subject of the Gentiles within Jewish thinking. A few, brief remarks about this are in order.
First, as Acts points out, the acceptance of Gentiles into the Christian community (of Jews) was difficult for most of the Jerusalem Jewish Christians. While certain of these Christians did speak with Gentiles about Jesus, Luke prefers to dedicate an entire chapter-and-a-half (Acts 10-11) to this thorny subject. The problem had not arisen in Jesus' life as it did in the early Christian community. Indeed, though Jesus was called 'a light for the Gentiles' by Simeon, he did not shine very often on Gentiles before his resurrection. Thus, it fell to the Christian community to deal with this problem. Peter, long acknowledged by Christians and Luke as the key figure among Jesus' closest companions, must undergo a conversion regarding the Gentiles. Acts 10 shows him ignorant and thus unwilling to deal with the pagan Cornelius; only by order of the Holy Spirit does Peter confront Cornelius, and, even when he first does this, he does not know what to say. Cornelius himself is equally in the dark about Peter; all he can say to Peter is, "we are all here in the presence of God to listen to all that you have been commanded by the Lord". When Peter says what he knows about God and Jesus, arriving as he does at the famous sentence, "anyone who believes in him will receive forgiveness of sins through his name", the will of God in regard to Gentiles is so clear to Peter that, though reluctant, he can only urge that these Gentiles, clearly forgiven by God and given His Spirit, should be baptized. Thus Luke explains the entry of the Gentiles into Christianity. In this form, the universality of Jesus' rule and power reaches beyond Israel to individual faith from all peoples. Not all Jewish Christians understood the acceptance of the Gentiles as did Peter and Paul, but the door was now never to shut on them
Second, while Peter said to Cornelius that Jesus was 'Lord of all', we read of Paul (at Athens; Acts 17) that he expected that all Gentiles now should hear the call of the only God to repentance and forgiveness through the resurrected Jesus. And indeed, long before Acts was written, Paul made such statement about Jesus that "in him everything continues in being", "absolute fullness resides in him", "reconciling everything in his person, both on earth and in heaven", "at Jesus' name every knee must bend". Many are the times when Paul speaks glowingly of the universal rule of Jesus, ruler of Israel as well as of the entire universe.
Third, that the Old Testament, which is essentially a story about Israel, begins with 11 chapters of non-Jewish material can seem mysterious until one realizes that the full truth about Israel is that she was God's third attempt by God to save all creation. God first tried with 'the first parents'; when that covenant failed, God tried to save everyone through Noah. When that covenant failed, God chose Abraham and his offspring and he said to them that they were to be the light of the world; i.e. they were to so shine in a world of darkness that this sad world would turn to find the true God. Israel always understood itself to be part of the plan of God to save all peoples, for He was King of all. It falls to the people of the 1st Century AD to realize just how God, through Jesus, was to realize the fullness of the divine desire that God rule over all peoples.
That Jesus is King of the Nations is the most natural of titles, for, as the New Testament witnesses, Jesus is King of the Nations even before the nations hear of him. It falls to us to make people know who is their loving King.