Lord Alton wrote the letter amid concerns that the government is accidentally overlooking Syrian Christians in wake of other vulnerable groups, such as gay people and moderate Muslims, because they are only taking refugees from United Nations refugee camps.
Christians often avoid the camps for fear further persecution, opting for Christian charities, homes or churches as refuge instead.
Advocacy groups like Aid to the Church in Need and Open Doors have said that Christians are the most persecuted group in the world at the moment.
Mr Alton said in his letter: "You will know of the considerable weight of evidence of assassinations by ISIS of Church leaders; mass murders; torture, kidnapping for ransom in the Christian communities of Iraq and Syria; sexual enslavement and systematic rape of Christian girls and women; forcible conversions to Islam; destruction of churches, monasteries, cemeteries, and Christian artefacts; and theft of lands and wealth from Christian clergy and laity.
"Christians and other religious minorities like the Yazidis face genocide and the UK should take steps to recognise that by offering those who cannot return to homes occupied by ISIS a route to safety in Britain."
Home Office Minster Lord Bates responded to the letter by saying the government would not specifically help Christians over anyone else, and that the United Nations are aware that some Christians purposely avoid their refugee camps out of fear and are consciously trying to find them and encourage them to register with their programme.
However Lord Alton said that Lord Bates' response was ironic because by choosing to treat all parties equally it overlooked those who are the most in need.
He said after receiving Mr Bates reply: "... a campaign of total annihilation is underway, which is why these minorities should be discriminated in favour of.
"Of course we should discriminate in favour of those most at risk, not least because we have a duty to protect the most vulnerable.
"Instead of creating a false even-handed dichotomy British policy should actively search out, and be weighed in favour of, those who are suffering the most: those who are on the receiving end of the very worst of ISIS's campaign."
