Taliban gunmen killed Shahbaz Bhatti, Pakistan's only Christian cabinet member at the time, in the capital Islamabad on March 2nd 2011. The militant group left a letter at the scene saying they murdered Shahbaz Bhatti because of his opposition to Pakistan's blasphemy laws.
These laws make it illegal to desecrate the Quran or speak ill of the Prophet Mohammad, which can lead to life imprisonment or death. The laws cover other forms of blasphemy against Islam as well.
International Christian Concern (ICC) says police have arrested all of the people involved in Shahbaz Bhatti's murder, including the two people who shot him.
However their trial still has not finished four years on, and ICC says this is because of threats from extremists. For example, the brother of Shahbaz, Paul Bhatti, was forced to flee Pakistan because of threats.
ICC say that according to media reports, Pakistan's federal government is considering moving the trial of Mr Bhatti's murder to a closed military court, which would speed up the process and reduce interference from radicals.
However this kind of court would not be open to the public, which is a fundamental part of the justice system in many countries.
ICC's Regional Manager for South Asia, William Stark, said: "Blasphemy laws continue to be one of the greatest threats to Christianity in Pakistan. While blasphemy laws claim to seek religious harmony through uniformity, in practice they provide cover for the pursuit of personal vendettas and crush fundamental freedoms for Pakistan's religious minorities.
"By protecting these laws and failing to bring Shahbaz Bhatti's killers to justice, the Pakistani government is further emboldening extremists to commit ever-more violent acts against religious minorities.
"ICC calls on the Pakistani government to bring Bhatti's murderers to justice as soon as possible.
"Moreover, ICC calls for the release and safekeeping of Asia Bibi, whose blasphemy accusation and conviction has brought about so much loss already."