We read in Luke 22:34 that Satan had to ask permission to sift Peter through temptation. Notice that Jesus not only said Peter would return, but that he would also encourage others.
Peter was not the only disciple who forsook Jesus. They all did (John 16:31-32). Peter was not meant to wallow in regret the rest of his life. He was instructed to use the good news of his restoration to encourage others who had failed.
