During the absence of Imam Al-Mahdi (aj.), we must stand up for those being tyrannized and oppressed, and those whose rights are being seized away.
It does not matter whether they are Muslims or non-Muslims. It goes without saying that we must stand up against atheism; however, if atheists were being oppressed, then it is our mission to restore their rights back.
Some Hadiths mentioned by the Household Members (a.s.) relate that: “Allah, the Most Exalted, enjoined one of His prophets living in a country under the rule of a tyrant: ‘go to this tyrant and tell him that I did not make use of you to shed the blood of the people and gather wealth, but rather to soothe the cries of the wronged, because you should not neglect their plight even if they were disbelievers.’” Allah does not want to wrong the unbelievers, just as he does not want us to wrong the Muslims.
He wants us to give every human his rights, whether close or not, and whether weak or strong, and this is the true meaning of justice.
In conclusion, it is our mission to stand up for the oppressed against the oppressor, be he a Muslim or a non-Muslim. To oppress is to exercise an authority in an unjust manner, which is definitely rejected by Allah: “That future abode, We assign it to those who have no desire to exalt themselves in the earth nor to make mischief and the good end is for those who guard against evil.” (12:83).
http://english.bayynat.org/infallibles/infallible_Philosophy_Mahdi.htm#.Wm4ZddFOmf0
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