How can the Muslim be a trial for the kaafir?

He can be a trial for those who disbelieve if the kuffaar gain power over him, because the kaafir’s gaining power over the Muslim is a trial (fitnah), as they will carry the burden of the sins they commit against us, so that is a trial for them.  

The phrase “a trial for those who disbelieve” may also be understood as meaning that they try to tempt us away from our religion by means of persecution, as in the verse in which Allaah says (interpretation of the meaning):

“Verily, those who put into trial the believing men and believing women (by torturing them and burning them), and then do not turn in repentance (to Allaah), then they will have the torment of Hell, and they will have the punishment of the burning Fire”

[al-Burooj 85:5]. 

This verse includes both meanings, because the two meanings do not exclude one another. The basic principle in tafseer is that if the verse may be interpreted in two ways that do not exclude one another, then it must be interpreted in both ways. End quote. 

Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him).