Why does Paul speak of Christ sitting at God's right hand as past tense while Psalms spoke of it as future?
Paul writes of Christ sitting at God's right hand as past tense because the event is now accomplished. Psalms had spoken of it in the future, but by Paul's time the prophecy was fulfilled: Christ purged sins, arose, and sat down at the Majesty on high.
Les explains that Psalms looked forward to this session at God's right hand as future prophecy. But the same Creator and Sustainer of the universe, who upholds all things by the word of His power, by Himself purged our sins. He died on Calvary's cross and arose from the dead. Then, having purged our sins, He sat down on the right hand of the Majesty on high. Therefore Paul can write of it in past tense, because it is now accomplished and prophecy in that regard is already fulfilled.
The LORD said unto my Lord, Sit thou at my right hand, until I make thine enemies thy footstool.