Truth has a way of lodging in our hearts (even if we don't want it to stick) so that God can use it to challenge us in the future.  When Jesus cursed the fig tree, as a parable of how faithless Israel would be judged (Mark 11:13-14), Peter remembered.  When they went down the same road the following day, he looked out for the tree. The text indicates his surprise that it had withered overnight.

Peter remembered Jesus speaks with authority.  He wondered what would happen and yet was amazed that God's Word had such immediate effect!  Why should Peter react like this?  He had seen the blind receive new sight (Mark 10:51-52) and paralysed men walk (Mark 2:9-12); Jesus even raised dead people to new life (John 11:43-44).  But this was different.  It was one thing to witness divine healing, but quite another to see the effect of the wrath of God at close quarters.  Jesus' trainee apostles had to understand that both salvation and judgement come by the Word of God.

Jesus came to bring salvation through His sufferings on the cross, and to equip apostles to proclaim the gospel and announce the kingdom of heaven.  In His first recorded sermon in the synagogue in Nazareth, Jesus read from Isaiah 61:1-2, "The Spirit of the Sovereign Lord is on me, because the Lord has anointed me to proclaim good news to the poor.  He has sent me to bind up the broken-hearted, to proclaim freedom for the captives and release from darkness for the prisoners, to proclaim the year of the Lord's favour …".  But He omitted the next phrase, "… and the day of vengeance of our God".  He consistently taught that He had not come to judge the world but to save it from the judgement to come (John 12:47).  The only people He warned of judgement were the hypocritical religious leaders (Matthew 3:7-12).

So Jesus commanded Peter to believe in the wrath of God as well as His love.  It was an essential part of the gospel message, and of his training to be a church leader.  He remembered the lesson well because 1 Peter 4:5 says, "… they will have to give account to him who is ready to judge the living and the dead."  The whole reason Jesus died was to save us from the coming wrath (Romans 5:9).  If God's anger against sin is not real, there is no reason for the sacrifice of Christ, or for His incarnation.  Without the certainty of God's judgement, there is no incentive to proclaim the gospel.  That is why many fair-weather believers have no instinct to persuade others of the saving gospel, because they think God will include everybody in His heaven.  No.  Jesus had acted out the truth on the fig tree, and Peter realised that He was serious.  We need to believe in the real God too - the God whose terrible judgement is as certain as His wonderful love. Then gospel instincts can grow in our hearts.