Question:
Regarding Revelation 3:21: Why do those who overcome sit with Christ on His throne? What does it mean? Is this talking about judging or ruling with Him?

Answer:
All overcomers will sit with Christ on His throne because God’s grace has purposed us for this blessing. All believers are overcomers (1 John 5:4,5) and we are taught in Scripture that presently we suffer with and for Him, but when He comes in His glory to reign over the earth, we shall reign with Him (see Romans 8:17; 2 Timothy 2:12).

Presently Christ is sitting on His Father’s throne in Heaven (Ephesians 1:20; Hebrews 1:3; Revelation 3:21); etc.), but when He comes in power and glory to set up His kingdom, He will sit upon His own throne (Matthew 19:28; 25:31). He will sit there as King of kings and Lord of lords, and by His grace He will associate us with Himself.
Just as Eve shared dominion with Adam, so we will share dominion with Christ over His universal kingdom. I do believe this is speaking then of “ruling” with Him. He will give us positions of authority in His kingdom and these will be determined by our faithfulness to Him in the present age of suffering and overcoming. (The judgment seat of Christ for believers—1 Corinthians 3:10-14; 2 Corinthians 5:9,10—will reveal all that was done for Christ and the rewards received will be manifested when we return with Christ to reign.) But we must ever remember that it was His grace which gave us to believe and to become overcomers, and thus when the crowns for faithfulness are received there will be a casting of the crowns before the throne with these words on our lips, “Thou art worthy, O Lord, to receive glory and honour and power: for Thou hast created all things, and for Thy pleasure they are and were created” (see Revelation 4:10, 11).