Therefore being exalted to the right hand of God, and having received from the Father the promise of the Holy Spirit, He poured out this which you now see and hear.
“For David did not ascend into the heavens, but he says himself:
‘The Lord said to my Lord,
“Sit at My right hand,Till I make Your enemies Your footstool.”’“Therefore let all the house of Israel know assuredly that God has made this Jesus, whom you crucified, both Lord and Christ.” (Acts 2:33-36)
As Peter prepares to wrap up his first sermon, he again looks to David to confirm the identity of Jesus. Sitting at the right hand of God, Jesus is both Lord and Christ - the Anointed One. This passage from the Psalms (Psalm 110) is a recognition of the Messiah's sovereignty. It stands out as a reminder -- both to the Jews of Peter's day and the Christians of ours -- that God's chosen does things as He sees fit, whether those actions conform to our own ideas or not.
Remember, Israelites longed for a political Savior to free them from the tyranny of Rome. In bringing up Psalm 110, Peter was letting them know -- perhaps reminding them -- that God is at work on a much grander scale.
The passage serves to remind us, too, that though He is our personal savior, Jesus is so much more. He is a friend that sticks closer than a brother... but He is also King. He is the Christ, but He is also Lord. We who say we serve Christ would do well to remember that. This isn't simply a happy-go-lucky, skipping hand-in-hand friendship with Jesus. This Christianity is nothing less than service to the King of Kings.