We often discuss the irony of Paul's call to bring the gospel to the Gentiles inspite of his former position as a Jewish zealot who killed Christ followers. We are in awe of how God used Paul after all he had done prior to believing in Christ. However, what we do not discuss as much is that Peter, a disciple of Christ chosen to spread the gospel to his own people, also had a significant rap sheet even as a believer. Not only was his faith a bit shaky, he also denied Christ three times even after the time he'd spent ministering with Him.
Reflecting on when Jesus told Peter he would deny him, it's important to note that He didn't tell Peter to deny Him he just said that he would. God's foreknowledge of our sinful actions does not exempt us from the consequences or our responsibility of those actions, but our love for God and desire to live for Him does not eliminate our sinful nature either. Peter loved Jesus and didn't want to deny Christ, but he did so anyway. Paul writes in Romans 7:18-20 "And I know that nothing good lives in me, that is, in my sinful nature. I want to do what is right, but I can't. I want to do what is good, but I don't. I dont want to do what is wrong, but I do it anyway. But if I do what I don't want to do, I am not really the one doing wrong; it is the sin living in me that does it.
God, who is rich in love and mercy, STILL used Peter to bring many to salvation through faith in Christ. I believe God uses Peter's and Paul's differing stories to show that God's mercy extends beyond our past sins to our present and future sins as well and can use us to spread His truth despite them. The apostle Paul gets a lot of attention for spreading the gospel to the Gentiles due to his complete 180° transformation, but I think what God did through Peter after a blatant triple denial of his master displays the love and mercy of the Father even more.
God does not cause us to sin or lead us into sin, but He allows situations in our lives to show us our sin. Our nature to try and satisfy or protect self reveals our need for Christ when we inevitably come up short. No thing on this earth will ever compare to a relationship with Him and he will use whatever means neccessary to make that known. According to Romans 6, we should not continue in sin so that God can display more of His grace, but His grace and love does abound out of our sin shedding much light on the goodness of the gospel.