Revelation 7:1-8 lists 12 tribes of Israel sealed by God, 12,000 from each to make a 144,000 in total.  Notably however, the list omits Dan and is replaced by Manasseh, so that Joseph and Manasseh are in the list, but Ephraim (Manasseh’s brother) is not.  Of Joseph’s two sons, Ephraim was greater than Manasseh, (and sometimes bore the name of Joseph), so in this list Joseph no doubt refers to Ephraim.

So why is the tribe of Dan missing? The Bible doesn’t give an explanation.  However, the only indication we have is Dan’s association with idolatry.  Here are two examples. 

The first is Judges 18:14-31, which gives an account of Dan’s idolatry which continued “until the day of the captivity of the land,” several 100 years later (verse 30). 

The second is 1 Kings 12:29–30, where under King Jeroboam, Dan became one of the centres for golden calf worship.

To understand the significance of idolatry we first need to step back a bit and see why there is a group of 144,000 in the first place.

Rev 7:2  Then I saw another angel ascending from the east, having the seal of the living God. And he cried with a loud voice to the four angels to whom it was granted to harm the earth and the sea,
Rev 7:3  saying, “Do not harm the earth, the sea, or the trees till we have sealed the servants of our God on their foreheads.”
Rev 7:4  And I heard the number of those who were sealed. One hundred and forty-four thousand of all the tribes of the children of Israel were sealed:

Just before the earth, sea, and trees are harmed, 144,000 servants of God from the tribes of Israel are sealed, but the tribe of Dan misses out on it.  Rather than being literal, this is more likely symbolic to show that idolaters are not in this group of 144,000.  This is supported by the sin of idolatry being listed as a reason for not inheriting the kingdom of God.

1Co 6:9  Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived. Neither fornicators, nor idolaters, nor adulterers, nor homosexuals, nor sodomites,
1Co 6:10  nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners will inherit the kingdom of God.

Eph 5:5  For this you know, that no fornicator, unclean person, nor covetous man, who is an idolater, has any inheritance in the kingdom of Christ and God.

Rev 21:8  But the cowardly, unbelieving, abominable, murderers, sexually immoral, sorcerers, idolaters, and all liars shall have their part in the lake which burns with fire and brimstone, which is the second death.”

Conclusion

Why is the tribe of Dan missing?  The indication is that it is because the tribe of Dan was well known for committing idolatry.  It is a symbolic emphasis that those who commit idolatry will not be sealed.