I always try to imagine the reaction of the Apostles as Our Lord ascended into Heaven. Were they standing there with their mouths open in amazement, which is why they were staring into the sky even after He disappeared? As they stood there, they were left with a promise that Our Lord, who reigns in the Kingdom of Heaven and opened the way to Heaven for us, will return again to judge us worthy to enter into that kingdom.
At the time of Jesus' life on Earth, the Jewish people were awaiting a Messiah, a king who would save Israel from the oppressing nations. They thought that this would be an earthly king who would rise up and banish conquering nations from the land of Israel and reunite the tribes. Before Jesus, there were many who claimed to be this Messiah, and some even managed to build a small army before being crushed by the Roman military.
We now know who this Messiah was: Our Lord Jesus Christ, who was not merely an earthly king that established an earthly kingdom for the Israelite people, but a divine king who established a divine kingdom for all peoples of every nation. As we see in the first reading, the Apostles didn't have the benefit of the hindsight that we now enjoy when they asked if Our Lord would restore the kingdom of Israel. They were still thinking that the Messiah would establish an earthly kingdom, even after seeing Him tortured, crucified, buried, and then being with Him after He rose again.
They did figure it out eventually, as we see from St. Paul's letter to the Ephesians. He tells us that God “[raised Christ] from the dead and [seated] Him at His right hand in the Heavens”. (Eph. 1:20) Jesus was given the position of authority within the Heavenly kingdom, and was also placed as “head over all things to the church, which is His body”(Eph. 1:22-23), thus establishing the divine kingdom, the Church, over which Our Lord reigns from Heaven. As we recite every week in the Nicene Creed, “he ascended into Heaven and is seated at the right hand of the Father.”
In the Catechism of the Catholic Church, the Ascension is described as “the definitive entrance of Jesus' humanity into God's heavenly domain”. (CCC #665) That complicated sentence simply means that Jesus, who is fully human as well as fully divine, entered into heaven and opened the gates of Heaven to all humanity. Just as a king who was victorious in defending his kingdom might lead his troops in a triumphant parade back into the capital city, Our Lord leads the way into Heaven, showing us that it is possible for humanity to enter into Heaven.
While Jesus led the way to Heaven as the triumphant king, we have the promise of the angels in the first reading from the Acts of the Apostles that He will “return again in the same way”. (Acts 1:11) One of the most important tasks of a king was to be the final judge of any legal matter in the kingdom. The king set the laws and also enforced and ruled on them. Our Lord, as head of the Kingdom of Heaven, “will come again to judge the living and the dead”, as we also say in the Nicene Creed. At the end of time, which could be a week from now or could be 1000 years from now, Our Lord will return to Earth and we will experience the final judgment. In this judgment, we will be judged whether or not we are to enter into the Kingdom of Heaven and join Our Lord in the joys that come in the presence of God, our Heavenly Father.
Of course, we know that this is not an arbitrary judgment, but rather is a determination whether or not we followed His commandments. Our Lord even tells us before He ascends what we need to do in order to be judged worthy of the Kingdom: “Whoever believes and is baptized will be saved; whoever does not believe will be condemned.” (Mark 16:16) Note that merely being baptized isn't enough, nor is believing in Him without receiving the Sacrament of Baptism. Likewise, this must be an active faith, one that moves us to proclaim the Gospel and to reach out to those in need.
Our Lord Jesus Christ has ascended into Heaven, and now reigns at the right hand with God, our Heavenly Father. When Our Lord returns again for the final judgment, may He find us worthy and lead us triumphantly into the Heavenly Kingdom.