Salam Aleyku. In a saheeh narration it says that our Prophet (PHUB) said: "I am al-Mahi through whom Allah obliterates unbelief". Does that refer only to judaism or to christianity as well?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Among the names of the Prophet is "Al-Maahi" (the eliminator or eradicator) as in the Hadeeth which reads:

"I am Muhammad and Ahmad. I am Al-Maahi through whom Allah will eliminate disbelief. I am Al-Haashir after whom people will be resurrected, and I am also Al-‘Aaqib after whom there will be no other Prophet." [Al-Bukhaari and Muslim]

An-Nawawi said when interpreting this Hadeeth:

"The scholars stated that what is meant is eradicating disbelief from Makkah and Al-Madeenah, and the rest of the Arab countries as well as what was displayed to him from the earth and he promised that the power and control of his nation will reach. Al-Qaadhi 'Iyaadh said: 'It might also mean a general eradication, in the sense of having dominance by establishing the evidence and making the religion manifest, as Allah Says (what means): {…and the religion of truth to manifest it over all religion.} [Quran 9:33]'"

This includes the disbelief of the Jews and Christians, as Allah eradicated with him the disbelief of Jews and Christians who were in the Arabian Peninsula and in other countries, such as the Levant and in every place where Islam had spread. Also, his evidence has been established and manifest over the Christians, the Jews and other polytheists.

For more benefit, please refer to Fatwa 90915.

Allah knows best.