First, this passage shows that the gospel is a testimony of the revealed salvation which had been formerly promised to the ancestors through the continuous succession of generations. It also points out a distinction between the promises which kept the hope of the faithful in suspense and this joyful message, by which God declares that he has accomplished those things which he had formerly required them to hope for. In the same manner he states a little afterwards, that in the gospel the righteousness of God is openly manifested, which was testified by the Law and the Prophets. The same apostle calls it, in another passage, an embassy by which the reconciliation of the world to God, once accomplished by the death of Christ, is daily offered to human beings.
Second, Paul means not only that Christ is the pledge of all the blessings that God has ever promised but also that we have in him a full and complete exhibition of them, as he elsewhere declares that "all God's promises in him are Yes and Amen." And, indeed, the freely bestowed adoption, by which we are made children of God, as it proceeds from the good pleasure which the Father had from eternity, so it has been revealed to us that Christ (who alone is the Son of God by nature) has clothed himself with our flesh and received us as his brothers. That satisfaction by which sins are blotted out, so that we are no longer under the curse and the judgment of death, is to be found nowhere else than in the sacrifice of his death. Righteousness and salvation and perfect happiness have been established in his resurrection.
The gospel, therefore, is the promulgation of the Son of God manifested in the flesh, to renew a ruined world and to restore human beings from death to life. It is justly called a good and joyful message; in it perfect happiness is obtained. Its goal is to establish the kingdom of God in us, destroying the corruption of the flesh and reviving us by the Spirit, in order to draw us into heavenly glory. For this reason it is often called the kingdom of heaven, and the renewal to a blessed life, which is brought to us by Christ, is sometimes called the kingdom of God. Commentary On A Harmony Of The Gospels: The Argument." pg. 1,2 "Luke" NT III [Reformation Commentary On Scripture] Edited By Beth Kreitzer