God's Plan of Salvation
When God sent His Son to this earth, it was for the purpose of fulfilling the terms of the first covenant so a second and better covenant could be established.The first covenant proved that mankind could not keep God's laws perfectly and therefore could not earn his salvation by what he did alone. Salvation clearly was going to require God's grace.
God extended grace and mercy to man by allowing the sacrifice of His Son upon the cross. By grace we are able to receive eternal life, which we do not deserve and cannot earn. By mercy we are able to escape punishment, which we deserve because of our sins. God's grace gives us what we cannot give ourselves, forgiveness of sins.
The new covenant that God has made with man is one that requires man work out his salvation by choosing to follow Christ or to reject Him and His teaching. In order to become a part of this covenant one must belive in Jesus as the Son of God, submit their life to Christ and submit to the terms of the covenant
When presented with the opportunity to be a part of the new promise on the day of Pentecost, people asked what they should do. They were given a choice to "change your hearts and lives and be baptized, each one of you, in the name of Jesus Christ for the remission of sins. And you will receive the gift of the Holy Spirit." About 3,000 people, having faith in what they heard, were baptized.
Today we have the same opportunity to change our lives. God offers us the gift of salvation, if we have faith, are obedient to the "law of Christ," and are willing to live the life of a Christian.