Ezekiel 9:1 & 2

"Then I heard him call out in a loud voice, 'Bring near those who are appointed to execute judgment on the city, each with a weapon in his hand.' And I saw six men coming from the direction of the upper gate, which faces north, each with a deadly weapon in his hand. With them was a man clothed in linen who had a writing kit at his side. They came in and stood beside the bronze altar." NIV translation

Because of the indictment of the actions of the Israelites that was handed down in chapter eight, we see that a judgment is coming. God calls for the executioners and we see six angels taking the form of men. They all carry weapons to execute God's judgment. The seventh "man" is another angel and we see that instead of carrying a weapon he is carrying a pen for writing.

Ezekiel 9:3 & 4

"Now the glory of the God of Israel went up from above the cherubim, where it had been, and moved to the threshold of the temple. Then the Lord called to the man clothed in linen who had the writing kit at his side and said to him, 'Go throughout the city of Jerusalem and put a mark on the foreheads of those who grieve and lament over all the detestable things that are done in it.'" NIV translation

The glory (presence) of God is seen leaving the mercy seat and heading to the door of the temple.

This is a reminder that God's mercy is infinite in its ability but not infinite in time. Judgment was imminent for Israel and it is just as imminent for all of the world today. 

We see that prior to this judgment, the angel dressed in linen is sent to walk through Jerusalem and mark those that did not take part in the idolatry. This is very similar to what John describes in Revelation 7. The writing kit would have included a type of ink which is a picture of the coming blood of Jesus Christ. This mark would have been the letter taw which looked like an "X".