Satan asked permission from God to sift Peter as wheat. And God permitted him to do so - because Peter had a more important ministry than all the others who were not thus sifted. Jesus only prayed that Peter's faith would not fail. Peter denied the Lord thrice. But, as a result, he was thoroughly broken and humbled and wept bitterly and repented. Thus God's plan was accomplished and all the chaff of pride in him was sifted away. God used Satan to accomplish that. That is one reason why God has not destroyed Satan as yet. Praise the Lord.
After the resurrection, God sent a special message through the angel at the tomb saying, "Go and tell His disciples and Peter that Jesus is risen and going in front of you" (Mark 16:7). That phrase "...and Peter...." is so typical of our Lord. Wasn't Peter also a disciple? Why did the Lord make special mention of him? Because Peter was the one who might have felt that the phrase "His disciples" could not possibly include him any longer, because of his terrible failure. So that phrase was meant to tell him that the Lord still considered him as His apostle.
But in spite of that message, Peter was still so discouraged that he decided to go back permanently to his old profession of fishing (John 21:3). So the Lord went and called Peter back to his apostleship, personally. Such is the love of the Lord. He will keep on coming after us. It was thus that Peter came back and his "faith did not fail". It was not because Peter never failed or made a mistake.
So, if we are truly born again and have become disciples of Jesus, we must believe that God has accepted us in Christ and loves us unconditionally - because God IS LOVE. There are two types of believers:
(1) Those who believe that their heavenly Father's love is unconditional .
(2) Those who believe that their heavenly Father's love is conditional .
Those in Category 1 will be at rest because they believe the truth. The others will remain in perpetual unrest because they believe they must perform well in order for God to continue to love them. Every false religion in the world teaches that God's love is conditional - that His love is "performance-love" - a love that is based on how well we perform! Jesus came and taught the opposite. Yet most believers are still heathen in their thinking. We must expose Satan's lie. Because the truth is that God loves His children unconditionally. We can reject that love and fall away from God and be lost. But His love for His children is still unconditional. The story of the father's love for the prodigal son illustrates this perfectly.
It is true that there are different degrees in God's love for people. God loves all the people in the world to a certain degree (John 3:16). But God loves His born-again children to a greater degree. And among His children, He loves some more than others, because they fulfill certain conditions - as Jesus said in John 14:21 : "He who has My commandments and keeps them is the one who loves Me; and he who loves Me will be loved by My Father".And beyond those, God loves to the highest degree, those disciples who have forsaken everything to follow Jesus - God loves them as much as He loved Jesus Himself (See John 17:23).
But yet God's love itself is unconditional.
One distinguishing mark of God's love is expressed in Luke 6:35 as "expecting nothing in return". Human love always expects respect, love and gifts in return. But Divine love expects nothing. It has no inward demands on those it loves. God makes the sun to rise equally on the good and the evil, and is kind and merciful to the evil and the ungrateful. Only if we live in this Divine love can we be saved from all Phariseeism.