The mark of Cain - Genesis 4:15
The mark of the blood of the Passover Lamb- Exodus 12:7
The mark of the Beast- Revelation 13:16-18
The mark of the Stigmata (the Risen Jesus' crucifixion scars) - John 20:27-28

God's people have always been marked and set aside.  When Cain, in a fit of jealousy and anger, committed the first fratricide, God confronted Cain with the crying out of his brother's blood from the ground. Though Cain was under the curse of his brother's blood, God still had mercy on Cain and marked him so that others would not kill him.

When the people of Israel were slaves in Egypt and God rained down plagues on Egypt due to Pharaoh's refusal to set the Israelites free, He instituted the Passover.  The firstborn of Egypt were marked to die, but God made a way for His people to escape the curse.  The Angel of Death passed over the homes that had the mark of lamb's blood over the door frame- those people who were marked and set aside as God's people were spared- but he took the firstborn of the unmarked households.

The book of Revelation is quite a frightening read at first glance, especially if it is taken literally (and this is from someone who used to really enjoy horror novels and slasher flicks) but most of its rich imagery is meant as allegory rather than as a literal representation of apocalyptic events.  While I've never been one to focus too heavily on eschatology, it is no accident that the mark of the Beast relates to commerce and the acts of buying and selling.  Time and time again, when civilizations fail part of the pattern of unravelling includes corruption in the economy as well as the glorification of hedonism. 

Commerce in and of itself is not a bad thing.  Trade is essential to maintaining society, but there is incredible temptation for all humans to fall in to the trap of the pursuit of stuff, and to bow down to the false god of hedonism, when more is never enough.  Perhaps the "mark of the Beast" is an apt portrayal of the bloodthirstiness and absence of business ethics among the suppliers and traders and buyers of the world.  Perhaps as his hand was writing as the Spirit inspired his heart, John of Patmos was observing a world in which one had to go so far as to become Godless (to be marked with the mark of the Beast) and put aside all integrity and ethical principles in order to buy or sell. When Jesus overturned the moneylenders' tables He didn't have a problem so much with the exchange of money and practice of trade as He did with the shameless lack of ethics and profiteering that He observed in the temple. 

Jesus' crucifixion scars (the Stigmata) are the marks that cover it all.  I believe that Jesus came back from the dead marked so that those who saw Him would know where He had been and what He had endured so that we could be covered by His blood.  I have to identify quite a bit with Thomas.  I would have likely made the same demands of Jesus that Thomas did- "If that's where you've been I want to see the scars.  Prove it."  Talk is cheap, where's the evidence?  I know I would want to see the proof.

All people are marked, although it is up to us to live in accordance to the marks we accept.  In Christ, the mark of His Cross is freely given for the salvation of all people.  We, like Cain, have betrayed others and caused damage and death- but in God's mercy He marks us and says to us, "I will protect you even in your condition of exile. I will cover you even though you did nothing to deserve it." 

Our marks are visible to the world around us.  In Baptism we are marked with the Cross of Christ, but the challenge is to "put on Baptism as daily wear" to quote Martin Luther, and to "let our light shine before men, that they may see your good deeds and praise your Father in heaven." (Matthew 5:16)

Today it would really seem weird and macabre to slather blood from a recently killed animal across the door frame- though that would be a pretty obvious marking.  The evidence of the mark of the Cross of Christ is more subtle- a life characterized by the fruits of the Spirit rather than the worship of materialism, the hubris of self-centeredness, or the callousness of isolation. 

Thomas asked Jesus the question that the world asks us: Let me see your marks.  If this is who you are, if this is who you represent, then I want to see the evidence.