It has been authentically reported that the Messenger of Allah (ﷺ) said in a lengthy Hadeeth:

…a man would be a believer in the morning but end up as a disbeliever in the evening, or would be a believer in the evening but awaken as a disbeliever in the morning. He would sell his religion for a small worldly gain.

What is meant by disbelief in this Hadeeth, and how does a person sell his religion?

Indeed it has been authentically reported that the Messenger of Allah (ﷺ) said:

Hasten with deeds, before a Fitnah like fragments of a pitch black night arrives...

"Hasten with deeds…" Meaning with righteous deeds.  

…before a Fitnah like fragments of a pitch black night arrives, a person would awaken a believer in the morning but end up as a disbeliever in the evening, or would be a believer in the evening but awaken as a disbeliever in the morning. He would sell his religion for a worldly gain.

Muslim: Book of Eemaan; #169

What this Hadeeth means is that the strangeness of Islam would become so severe, to the point that a believer would awaken as a Muslim but end up as a disbeliever in the evening, and the opposite, one would be a believer in the evening and awaken as a disbeliever in the morning, selling his religion for a worldly gain. It happens by speaking something that is disbelief, or doing something that is disbelief for a worldly gain.

So this person would wake up as a believer, then someone would come and say to him; insult Allah, or insult the Messenger, or forsake the Salat and we will give you this and that. Say that fornication is permissible, say that alcohol is permissible, and we would give you such and such. So he would sell his religion for a worldly gain, and the evening or morning would come and he would be in a state of disbelief.

Or they would say to him; do not be with the believers, and we will give you such and such to be with the disbelievers, or with the party of the disbelievers. So they would tempt him into being with the disbelievers, a member of their party, one of the supporters, until they give him a great deal of wealth and he becomes a complete supporter of the disbelievers, and an enemy to the believers.

The types of apostasy are many, but the majority of them are because of a worldly affair; loving the Dunya and choosing it over the afterlife, this is why he said:

He would sell his religion for a worldly gain.

And in another narration;

Hasten with righteous deeds, are you waiting for extreme poverty that would cause you to forget everything else, or prosperity that would lead to corruption, or a grave that has been prepared for you, or an illness that would weaken you [so you would not be able to worship], or an old age that would deteriorate you, or the Dajjal (antichrist), for the Dajjal is a great evil who is waiting! Or are you waiting for the Hour, and the Hour is more disastrous and vicious.

At-Tirmidhi: #2228

So the believer hastens to preform good deeds, and is wary that he may be tested with an early death, an unexpected death. He may be tested with an illness that weakens him and prevents him from worshiping, or tested with old age or with something else. A person must take advantage of his life, health and intelligence by performing righteous deeds before a barrier prevents him from that; sometimes with a test such as an illness or otherwise, and sometimes by greed for a worldly affair, preferring it over the afterlife, and its beautification by the enemies of Allah and the callers to disbelief and misguidance.