In considering the historical development of God’s kingdom, John Frame writes,

Why [God] chose to stretch out the drama of salvation over so long a time is a mystery. The length of time is related to other mysteries in Scripture, such as the problem of evil. We would not cry, ‘How long O LORD?’ (Pss. 6:3, 13:1, 80:4; 90:13; Hab. 1:2; Zech. 1:12; Rev. 6:10), if God had determined to complete his purposes in an instant, and the sting of pain and suffering would be much less if God were to abbreviate his story to a few decades. But God’s decision is clear: that the history of redemption will take millenia, leaving space for dramatic movements, ups and downs, twists and turns, longing and astonishments. Salvation is to be a great epic, not a short story. God will glorify himself, not by measuring his kingdom in time spans appropriate to human kings, but by revealing himself as ‘King of the ages’ (Rev. 15:2 NIV).

– Frame, Systematic Theology, 88.