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.
Les teaches that the passage describes the same Creator and Sustainer of the universe upholding all things by the word of His power. When He had by Himself purged our sins, He sat down on the right hand of the Majesty on high. Les notes this is exactly what Psalms said, only Psalms spoke of it as future. Paul can therefore write of it as past tense, because it is now accomplished and prophecy in that regard is already fulfilled. Les then connects this to I Corinthians 15: the Kingdom has just been ushered in, everything is under the King's power, and Christ is Messiah of Israel, Son of God, Savior of mankind, and Head of the Body. Les emphasizes Christ is not the King of the Church but the Head of the Body; if believers were under a kingship, they could not be called joint heirs, because they would be subjects.
The LORD said unto my Lord, Sit thou at my right hand, until I make thine enemies thy footstool.