LUKEWARM

"So then, because you are lukewarm, and neither cold nor hot, I will vomit you out of My mouth."      Revelation 3.16 

When a Christian is filled with the Holy Spirit, sacrifices and lives by faith, he is hot. 
On the other hand, an unbeliever would be considered cold.
Although being cold is bad, it's still better than being lukewarm in the faith. This type of person is undefined about what he wants. Like the waves of the ocean, he does not know what he wants.

Wrong Path

"Take heed to yourselves, lest your heart be deceived, and you turn aside and serve other gods and worship them..." Deuteronomy 11:16

If we don't watch in order to protect our heart, we'll eventually be taken by this world's fascination, and with that, we'll grow cold in our faith, and eventually be distant from the Kindgom of God.

Iniquity

"And because lawlessness will abound, the love of many will grow cold." Matthew 24:12

The coldness of the heart you find too often nowadays is a result of the iniquity abound. When one lacks spiritual values, moral values are usually affected with little and almost no true love left, which by the way, can only come from faith.