This passage starts with a command, "Have faith in God"!  Jesus was teaching Peter, and the other trainee apostles, not to be surprised that God's words are followed by action.  In his future role as a leader of the early church, Peter would need to speak out God's Word - and expect that God would act powerfully.  The apostolic gospel was to be one of salvation through preaching (Romans 1:16).  The key is in the kind of relationship they were to have with Jesus Christ: if they trusted Him, He would act in accordance with their faith.

Although Jesus had commended the father of a demonised boy for his weak faith (Mark 9:24), He was now commanding total, unswerving and undoubting faith.  There was no need for tentative approaches to the throne of grace (Hebrews 4:16).  The Lord expected His team to go out with boldness and confidence.  When we are assured that our prayers are absolutely in line with His will - the answer is given in power.  James, the half-brother of Jesus understood this well and repeated it in His letter (James 1:6).  

But this truth can also be taken out of context by the Tempter who twists it to excite power hungry people, even in the church; or to depress those who have longed for healing but are still unwell.  'Believe that you have received it and it shall be yours' has become a formula for some, for others an unkind spiritual goad to accuse those whose prayers seem unanswered … that they have insufficient faith.  This is not a call to psyche ourselves up to believe something, but to take whatever quiet confidence the Lord gives us (which is in accordance with Scripture) and trust Him because we know that God has promised it - and we believe Him.  It is not our prayer that 'works', but God who works powerfully in response to our believing prayer.  If we believe, we will pray.  

Likewise, if we believe in God's forgiveness, we demonstrate our faith by forgiving others.  If we are indifferent to His forgiveness through the sacrifice of Christ, what sort of faith do we have?  If we cannot forgive others, why should the Lord forgive us or perform any lesser miracle?  Jesus taught the same principle in the Sermon on the Mount (Matthew 6:14-15).  Tepid praying mirrors lukewarm faith in tepid salvation with tepid forgiveness.  Believing, is monitored by the things that demonstrate faith - obedience and trusting prayer.  The old hymn has a good line, "Trust and obey, for there's no other way, to be happy in Jesus, but to trust and obey"!  That is the theory; now today is the day for the practical class!