We believers are no longer a part of Satan's kingdom and are consequently expected to disengage from the ways of the world. In doing so, we stand as a testimony to the light which the world in darkness hates. This is the source of animosity that leads to tribulation for the believer.
This is the judgment, that the Light has come into the world, and men loved the darkness rather than the Light, for their deeds were evil. John 3:19

If you were of the world, the world would love its own; but because you are not of the world, but I chose you out of the world, because of this the world hates you. John 15:19
Consequently, tribulation is something that a believer should expect to encounter. Jesus said that while we are in this world we would have tribulation, but to be courageous because He has overcome the world. His victory over sin and death is our assurance of eternal life, a hope beyond this world.

Jesus describes a time of unprecedented persecution of believers that takes place before His return. It is referred to as a time of great tribulation or the great tribulation and takes place when the fifth seal in Revelation 6 is opened. This is when the man of sin declares himself to be God and issues the ultimatum that whoever does not worship him is to be put to death. His effort to purge the world of believers is expedited by the requirement to receive his mark in order to buy or sell anything.