Jesus appeared to his disciples for forty days and “spoke about the kingdom of God.” (Acts 1:3) We can assume Jesus did not speak about the hope of the age to come for forty days—the disciples were quite familiar with that. Rather Paul tells us that this forty days was about the revelation “that Christ died for our sins” (1 Cor. 15:3). This atonemental revelation was given to Peter, James, the Twelve, more than 500 brothers (v. 5-7), and finally to Paul “as to one abnormally born.” (v. 8; cf. Acts 9) Paul also tells us that he received this revelation directly from Jesus (Gal. 1:12)—i.e. he did not go up to Jerusalem to receive it (v. 18).
The idea that the death of the Messiah is accounted by God as an atoning sacrifice in the stead of humanity’s sins is not something the apostles figured out. Rather, they received it directly from Jesus. Justification by faith in anticipation of the Day of the Lord is the “golden seed” of apostolic revelation, planted by Jesus for forty days (cf. Passover) and galvanized by the outpouring of the Holy Spirit. This pattern of receiving the message of the Cross followed by the gift of the Holy Spirit is seen throughout the book of Acts, and it is Paul’s logic to the Galatians: “Before your very eyes Jesus Christ was clearly portrayed as crucified. I would like to learn just one thing from you: Did you receive the Spirit by observing the law, or by believing what you heard?” (Gal. 3:1-2) The early church centered around the atonemental teachings of the Apostles (cf. Acts 2:42), and to this day those who are not “severed from Christ” (Gal. 5:4) hold fast to it as the only hope of eternal life.