It's important to get this one right. The Kingdom of God isn't another name for our popular understanding of heaven. That is, it's not where we go when we die. Nor is it even a "place" in the temporal sense of the word. Biblically, the Kingdom refers to God's rule rather than God's realm. Being Kingdom citizens is a matter of embracing God's will as our own and living accordingly.

This helps us appreciate why Jesus refers to the Kingdom as "among" us and even "within" us—always within reach if we but reach for it. We don't have to get there so much as abide there wherever we are. Scripture says we participate in the Kingdom's reality in various ways: repenting and changing our hearts, working toward justice, protecting the vulnerable, and freeing those who are burdened. 

Matthew speaks of "the kingdom of the heavens," while Mark and Luke prefer the more direct "kingdom of God." All three gospels see the fulfillment of the Kingdom as central to the teaching of Jesus. We're taught to pray for its arrival in the Lord's Prayer. Our relationship to money and even to family might be an obstacle to full admission. Jesus offers multiple parables and metaphors for understanding the Kingdom's dimensions and implications: a sower, a mustard seed, treasure, a banquet. Matthew's gospel alone references the Kingdom almost 50 times.

In John's gospel, Jesus is clear that the Kingdom can't be mistaken for territory gained by power: it operates distinctly from this world. It's a reality where peace rules and oppression ends. Jesus manifests the Kingdom by coming into this world, but its fullness is not yet in view until his return in glory.

Sometimes we make the mistake of morphing the Kingdom of God with the church on earth. At its best, the church is the sacramental sign of the Kingdom: a signpost, that is, not the destination. We the church proclaim the Kingdom both in formal preaching and in works of justice and mercy. No political system or social program can establish God's rule. We can't make "Kingdom come" by our own efforts. Yet we are summoned to cooperate with the Spirit to enter more fully into the Kingdom's reality by our personal choices and in reshaping society to conform to its values.

Scripture: 1 Chronicles 17:14, 28:5; Psalm 99:4; 146:5-10; Isaiah 6:1-5; 24:23; Zephaniah 3:15; Zechariah 14:16-17; Mark 1:15; 9:1; 10:23-25; Matthew 3:1-2; 4:17; 6:10; 10:34-38; 13:18-19, 24-53; 16:19, 28; 19:23-24; 20:1-16, 20-23; 22:1-14; Luke 9:27; 11:2; 14:15-33; 18:24-25; John 3:3-5; 18:33-37; Acts 8:12; 14:22; 19:8; 20:25; 28:23

Books: Parables of the Kingdom/Jesus and the Use of Parables in the Synoptic Tradition, Pts. I-II, by Mary Ann Getty-Sullivan (Liturgical Press, 2007 and 2008) 

A Banqueter's Guide To The All-Night Soup Kitchen Of The Kingdom Of God, by Patrick T. McCormick (Liturgical Press, 2017)

We regard time linearly because we experience it this way: each day marching in a progression toward the future. We view the past similarly, only fixed as if set in amber. It's natural to imagine the birth of Jesus as we do our own birthdays: stamped on the calendar on December 25th and celebrated on its anniversary annually. 

Except in the East, where Christ's coming is observed on January 6th. How can this be?

Consider that the ancients told time in terms of the great sky clocks, sun and moon. Seasons and occasions were established according to the heavenly orbs, not to mention patterns of rainfall. Calendars were shaped by familiar cycles so crops might be planted and harvested successfully.

Another significant factor in dating was the succession of rulers. So Matthew tells us about a new star in the heavens signaling a new king's arrival, but also that this occurred during the present reign of King Herod. Luke also acknowledges who held the reins of power during this event: Roman Caesar Tiberias, Palestinian procurator Pontius Pilate, and high priests Annas and Caiaphas. Details we might prefer—day of the week, date of the month, or month itself—weren't recorded. A study of celestial anomalies of this period hasn't produced an exact calendar date for the newborn king.

As the early church spread out across many cultures, news traveled slowly. Inevitably the church experienced natural divisions and differing customs. Churches of the East and West were both improvising. The winter solstice, the longest night of darkness around December 21, was already a significant observance. It seemed fitting to celebrate the arrival of the Light of the World as days grew longer beyond the solstice. Jerusalem tradition had chosen January 6 for the celebration of this Theophany, Greek for "Divine Manifestation," and this custom was embraced throughout the Eastern churches. Many of these churches observed a sequence of "epiphanies" leading up to the great one: annunciations to Zechariah and Mary, Mary and Elizabeth's visitation, John's birth, the annunciation to Joseph, and finally the birth of Jesus. 

Meanwhile, Western Christians called this same divine manifestation by the name Epiphany, observing it in late December. In time, Eastern and Western traditions mingled to create a unified liturgical chain between Nativity and Epiphany. Choosing a precise date for a historically unknown event was less important than preparing to receive this manifestation with minds and hearts fully awake.

Scripture: Exodus 23:14-17; Leviticus 23:1-44; Deuteronomy  26:1-3; 1 Maccabees 4:56-59; Ecclesiastes 1:4–7, 9-10; 3:1-8; Hosea 2:13; Zechariah 14:6-9; Mark 1:14-15; Matthew 2:1-2; Luke 3:1-2; John 1:1-5, 14

Books: Days of the Lord: The Liturgical Year, Vol. 1: Advent, Christmas, Epiphany (Liturgical Press, 1991)

Biblical Meditations for Advent and the Christmas Season, by Carroll Stuhlmueller, CP (Paulist Press, 1980)