Why does God allow Satan limited dominion upon this Earth?
God allows Satan limited dominion on this Earth and uses him to separate the faithful from the unfaithful; the wheat from the chaff; the sheep from the goats. God allows Satan’s evil to reign yet restrained by the Holy Spirit (2 Thessalonians 2:7) as the faithful angels in the Kingdom observe, over the previous 6000-years, the horror of evil and disobedience that will eternally act as a bulwark against future rebellion as free will remains extant in the Kingdom.
Our entire creation within the Realm of Time is the resultant of an unfathomable spiritual war, a rebellion, having manifest in the Kingdom of God mandating a neutral repository (Earth) for that spiritual war as the destructiveness of evil/sin is not permitted in the Kingdom (Revelation 21:27). God the Father extricated this evil from the Kingdom (Luke 10:18), commissioned the Son, Jesus Christ (John 3:35; John 1; Hebrews 1; Colossians 1), to take elements from the unseen Spiritual Realm and create matter, Time, our Universe (Hebrews 11:3), and is presently dealing with His enemies within the Realm of Time partially constrained by Time and physics (Genesis 3:15).
The primary reason the Son of God, Jesus Christ, appeared within the Realm of Time was “to destroy the works of the Devil” (1 John 3:8b). Satan’s limited dominion on Earth has eternal relevance in God’s eschatological Plan for the New Jerusalem going forward as free will remains extant as free will is essential for love and relationship to exist with purity and authenticity.