Traveling by air today requires that you first go through airport security and present your identification before you can board your plane. And when the TSA agents pull out a mysterious little light and run it over your driver's license, they are authenticating it. They are making sure that everything is legitimate. With that little light, they can find a mark that isn't visible to you. But they can see it with their light.
There is a mark on believers that God can see. He knows who belongs to Him. But during the Tribulation period, there also will be a mark on the people of the Antichrist (see Revelation 13:16–18).
When Christ comes into our lives, we are sealed with the Holy Spirit. The Bible says, "In Him you also trusted, after you heard the word of truth, the gospel of your salvation; in whom also, having believed, you were sealed with the Holy Spirit of promise, who is the guarantee of our inheritance until the redemption of the purchased possession, to the praise of His glory" (Ephesians 1:13–14).
In ancient days, when a king would send something, he would seal it. The seal from his signet ring, imprinted in the wax, essentially said, "This belongs to the king. Don't mess with it."
As a believer, you belong to the King. He has sealed you. He has marked you. You have his ID tag on you. You are his property.
An elderly gentleman who was known for his godly life was asked, "What do you do when you are tempted?"
He said, "I just look up to Heaven and say, 'Lord, Your property is in danger.' "
Do you have God's mark? Is His I. D. tag attached to you? Can you proudly say that you belong to Him?