Acts 2:36 -39
Therefore let all the house of Israel know assuredly, that God hath made that same Jesus, whom ye have crucified, both Lord and Christ.
Now when they heard this, they were pricked in their heart, and said unto Peter and to the rest of the apostles, Men and brethren, what shall we do?
Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost.
For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call.
Why did Peter refer to Jesus as both “Lord” and “Christ”? In Greek, the word for Lord is Kurios, meaning supreme in authority, by implication, Master (as a respectful title), God. The word for Christ is Christos, meaning the anointed one, i.e. the Messiah, an epithet of Jesus:--Christ.
Peter wanted to make sure they understood that this man Jesus that they had crucified was not just an ordinary man. The use of the word Lord emphasized the deity of Jesus. He was the One Israel had been waiting for, the promised Savior, the Messiah.
When they understood who it was that they had crucified, their hearts were broken over what they had done. Teaching people who Jesus really is has a way of making people see their sinfulness in a whole new light. Telling them who Jesus is tends to make them sorry for their sin, and makes it easier to lead them to repentance. That's what Peter was doing: making sure they knew who Jesus was.
These verses contain a message that is still relevant to us today, because, as Peter pointed out, the promise of the gift of the Holy Ghost was not just for them: it was for the following generations, even those who were far off. In other words, that promise is for everyone the Lord shall call. Still to this day, so far off from the time these words were first spoken, when a person is called by God and repents of their sins, they receive the promise of remission of their sins and the abiding presence of the Holy Ghost within them.
Take a moment to thank God for Jesus, who is both Lord and Christ, and in whom we have both forgiveness of sin, and the help and guidance of the Holy Ghost to help us along the way.