Jesus warned prospective followers of the dangers of following him. Potential Christians were cautioned that whoever could not take up their cross and follow him could not be a disciple. But wait just a minute - how could people understand what that meant? Jesus had not yet been crucified - why would he refer to something that had not yet happened? Or perhaps this saying was only attributed to Jesus after he had already been crucified.What? You didn't know that the Romans invented crucifixion just for Jesus? Seriously, they never nailed anyone to trees before 33ad.
Except for Caesar in the First Century bc*, who crucified a whole den of pirates who captured and then released him.
Except during the Third Servile War (aka the rebellion of Spartacus) when Caesar's buddy Crassus crucified 6,000 unlawful combatants along the Via Appia as a warning to the rabble of what happens whan one takes on the might of Rome.
Except for 30 years before Jesus spoke, when Roman consul Publius Quinctilius Varus crucified 2000 Jews right in the middle of Jerusalem**, one of the main causes of anti-Roman sentiment in the Jewish nation in Jesus' day.
Crucifixion was, in fact, probably the most common (and certainly the most visible and most feared) form of capital punishment in First Century Rome***. It was so loathed that Roman citizens were exempt from it, and because it was a form of punishment specific to the oppressed, the occupied, and the rebel, Jesus' Jewish audience had a perfect understanding of his words.
Too bad some modern detractors can't make the same claim.
* We all remember what that means, right?
** "Upon this, Varus sent a part of his army into the country, to seek out those that had been the authors of the revolt; and when they were discovered, he punished some of them that were most guilty, and some he dismissed: now the number of those that were crucified on this account were two thousand" - Josephus, Antiquities (17/10/x)
*** in fact, in the whole ancient world, being practiced in Greece, Assyria, Carthage, Egypt, and Syria, among other places.