…who is even at the right hand of God… – Romans 8:34 (see also: Hebrews 12:2)

Christ sits in heaven. Now, He never would sit if the work were not fully done. Jesus, when He was on earth, had a baptism to be baptised with, and how was He straitened until it was accomplished! He had not time so much as to eat bread, full often, so eager was He to accomplish all His work. And I do not, I cannot, imagine that He would be sitting down in heaven in the posture of ease, unless He had accomplished all-unless “It is finished!” were to be understood in its broadest and most unlimited sense. There is one thing I have noticed, in looking over the old levitical law, under the description of the tabernacle. There were no seats whatever provided tor the priests. Every priest stands daily ministering and offering sacrifice for sin. They never had any seats to sit on. There was a table for the shew-bread, an altar, and a brazen laver; yet there was no seat. No priest sat down; he must always stand; for there was always work to be accomplished, always something to be done. But the great High Priest of our profession, Jesus, the Son of God, hath taken His seat at the right hand of the majesty on high. Why is this? Because, now the sacrifice is complete for ever, and the priest hath made a full end of His solemn service. What would the Jew have thought if it had been possible for a seat to have been introduced into the sanctuary, and for the high priest to sit down? Why, the Jew would then have been compelled to believe that it was all over, the dispensation was ended; for a sitting priest would be the end of all. And now we may rest assured, since we can see a sitting Christ in heaven, that the whole atonement is finished, the work is over, He hath made an end of sin. I do consider that in this there is an argument why no believer ever can perish. If he could, if there were yet a chance of risk, Christ would not be sitting down; if the work were not so fully done, that every redeemed one should at last be received into heaven, He would never rest, nor hold His peace. ~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/0256.cfm