In the first part of today's Gospel reading from John 13, Judas' betrayal of Jesus is announced. At the Last Supper, when the disciples asked who the betrayer is, Jesus dipped and handed a morsel to Judas to indicate that he would be the one. In verse 27, we heard read, "After Judas took the morsel, Satan entered him." Satan becomes a partner in crime when one premeditates committing a sin, and that sin will be serious.
In the last part of the Gospel, Peter's denial of Jesus is predicted. Jesus told Peter that the cock would not crow before he denied him three times. Peter's denial stemmed from fear and came out spontaneously, not premeditated as in Judas' case, and he repented right afterwards. This is the difference. The goal of union of God is that we don't even commit spontaneous sins.