Whoever does not take up his cross and follow after me is not worthy of me.
We may not be too astonished by this today -- my own parish church, for example, has a twelve-foot high crucifix hanging from the ceiling above the altar, in case anyone forgets what happened.
But, in Matthew's telling, this is part of the instruction Jesus gives the Apostles before sending them out to preach that the kingdom of heaven is at hand. My guess: they weren't thinking, as they accepted authority over unclean spirits and diseases, that the kingdom of heaven was going to be founded upon a crucifixion.
The first mention of the cross in Matthew, explicitly that of the disciple, but implicitly that of Jesus (and follow after me). Crucifixion was a form of capital punishment used by the Romans for offenders who were not Roman citizens.
I knew that about crucifixion being reserved for non-citizens, but only now have I noticed the spiritual implication of this most secular law: those who are worthy of Jesus are not citizens of the world, and the world knows this.