Yes, our citizenship is in heaven, but as Jesus shows His Apostles and each of us, we can't attain that citizenship without the cross. Jesus had His Cross; we each have our own.
Jesus' cross was always before Him. He tried to prepare His Apostles for what was soon to come. They, like me, don't always want to hear what my Lord is saying. I sometimes find myself saying to Jesus, "You can't be serious. You really want me to go through this?!"
Just as I think Jesus gave those three Apostles the privilege of that mountaintop moment to help them bear what was to come, He gives me and each of us our own moments on the mountain. Whenever He does give them to me, I always pray: "Lord it is good that I have been here. Having been in Your Presence, may I not come down from the mountain the same way I went up, but reflect more of Your image."
When I look at my life, I can see there have been crosses and there will continue to be crosses. The mountain top moments remind me of that citizenship that God has promised me. Those moments strengthen and restore me so that I can continue to carry the crosses He gives me.
During the Last Supper, as Jesus' Cross comes closer, He tells His Apostles that He goes to prepare a place for them, that in His Father's house there are many mansions. (John 14:1~4)
I guess it could be said that Jesus, in His Cross, had the "master key" that unlocked the gates of heaven. However, in order to gain entrance to that particular place He has prepared for me, I need to accept that cross~shaped key He offers.
So I will continue to travel the road, cross~key in hand, taking respite in those occasional mountain top moments.
See you on the mountain, and by God's grace, one day in heaven.