The good news of Christianity, or the gospel, is the truth that there is only one God who has eternally existed in three persons: Father, Son, and Holy Spirit.
He is holy, perfect and just, and He made all of us in His image to know Him and reflect Him.
But we sinned against God and cut ourselves off from His favor, and because of this we all deserve His eternal wrath in hell – eternal conscious torment.
But, in His great love, God sent His Son to become a man in Jesus Christ. Jesus lived a perfect life, died as a substitute on the cross for sinners, thus fulfilling the law and taking upon Himself the punishment for their sin.
He rose from the dead three days later, showing that God accepted Christ’s sacrifice and that God’s wrath against His people had been exhausted.
All men and women in all places are now called to turn or repent from sin and to trust in Christ alone by faith alone for forgiveness. If we repent of our sins and trust in Jesus Christ as Lord and Savior, we are born again into a new life, an eternal life with God.
Here’s an exposition Noah Braymen preached on Romans 1-3 titled What Is the Gospel?
The book, Tell the Truth by Will Metzger, is another helpful resource.