What does Paul mean by being crucified with Christ in Galatians 2:20?

Les explains that being crucified with Christ is the very function of the Gospel: Christ died for the sins of the world, was buried, and rose from the dead. When a person comes under that forgiveness, Les teaches, they are forgiven everything—past, present, and future—and will never come into God’s presence with sin on their back, because it has all been paid for. This is beyond human comprehension, yet it is believed because the Book teaches it. Forgiveness of all trespasses is not because of anything we have done, but because of what Christ accomplished on our behalf. Paul goes on to say that though he is crucified by identification with Christ’s death, yet he lives; the life he now lives in the flesh is lived by faith, and Les prefers the rendering “the faithfulness of the Son of God.”