Why can Paul write about Christ sitting at God's right hand as already accomplished?

Paul writes of Christ seated at God's right hand as accomplished because the event had already occurred: after purging sins by Himself, Christ sat down at the Majesty on high. What Psalms foretold in the future, Paul now records in past tense as fulfilled prophecy.

Les explains that the One who upheld all things by the word of His power is the same Creator and Sustainer of the universe who died on Calvary's cross and arose from the dead. Having by Himself purged our sins, He sat down on the right hand of the Majesty on high. Psalms had spoken of this in the future, but now Paul can write of it in past tense because it is now accomplished and the prophecy is already fulfilled. Les then connects this to I Corinthians 15: the Kingdom has been ushered in, everything is under the power of the King, who is the Messiah of Israel, the Son of God, the Savior of mankind, and the Head of the Body. Les makes the point that Christ is not the King of the Church but the Head of the Body, so believers can be joint heirs rather than subjects.

Psalms 110:1

The LORD said unto my Lord, Sit thou at my right hand, until I make thine enemies thy footstool.

Why does Paul speak in past tense about Christ sitting down at God's right hand when Psalms spoke of it as future?

Paul writes in past tense because Christ's purging of sins and sitting at God's right hand were already accomplished by Paul's time. Psalms spoke of it as future, but for Paul it was fulfilled prophecy. The same Creator and Sustainer of the universe purged sins by Himself, arose, and sat down on high.