4.26 The manner in which the Gospel includes, in substance, the books of the Old Testament
Moreover, by this word Gospel we are far from meaning what is commonly called such, i.e., certain extracts which are disconnected without reason, neither discourses from the books of the four Evangelists or from the Epistles of St. Paul. On the contrary, we understand under this word Gospel, not only all of the New Testament but also all that has been promised or predicted in the Old Testament on the subject of Jesus Christ (Acts 26:22, 23; 28:23; John 5:39; Rom 1:2).
For, as we have already said, the Gospel is the only means by which from the beginning of the world, God has always saved His elect (Heb 13:8; Acts 4:12). That is why, as Moses declares (Gen 3:15), God began to announce it to the world from the sin of Adam, although it was manifested and preached clearly, a long time afterwards, by Jesus Christ Himself in Person, and by His Apostles (Rom 1:1-6: 16:25, 26).
Thus, to summarize, we call Gospel the Good News which, from the beginning, and by His grace and mercy alone, God has announced to His Church: those who, by faith, embrace Jesus Christ shall partake of eternal life in Him (Rom 3:21, 22; John 6:40).