What was hard to understand for Peter was how a former blasphemer against the Holy Ghost could be forgiven even though it was unforgivable under the kingdom gospel (Matthew 12:31-32 KJV, 1 Timothy 1:13-16 KJV). Also, how Paul's message was of salvation by grace through faith (Romans 4:4-5 KJV, 2:8-9 KJV), that we are saved (1 Corinthians 15:2 KJV, 1 Corinthians 1:18 KJV, Ephesians 2:4 KJV vs. Peter's message/Israel's message was and will be that of doing something to be saved: baptism of repentance for the remission of sins (Acts 2:38 KJV), keep the commandments (John 14:15 KJV, Matthew 23:2-3 KJV, John 15:5-10 KJV), endure to the end to be saved (Matthew 10:22 KJV), go through the trial of their faith (1 Peter 1:7 KJV), receiving the end of their faith, even the salvation of their souls (1 Peter 1:9 KJV).