Why could God save people in Acts 2 based on the shed blood?
The finished work of the cross had been consummated, so God could now save those people based on the shed blood. At that point the cross was complete, enabling salvation on that basis, though the text does not yet indicate they believed the later Gospel message Paul defines in 1 Corinthians 15.
Les explains that Acts 2 continues the same message John the Baptist preached: when people repented, he baptized them, and they became a separated group of Jews. At Pentecost, the same pattern appears: those who gladly received His Word were baptized, and about three thousand were added. The difference, Les says, is that now the finished work of the cross had been consummated, so God could save these people based on the shed blood. Yet Les notes there is nothing yet indicating they believed what is later called the Gospel—1 Corinthians 15:1-4, that Christ died for our sins according to the Scriptures. Instead, the message was, “You killed Him.” These believers were added to the group and continued steadfastly in the apostles’ doctrine, the twelve, who knew mainly that Christ had come and fulfilled all the Old Testament promises.