Did the Prophet, peace and blessings be upon him, allow the delegation of Najran Christians to pray (Christian prayer) in the Prophet's Mosque, and is it permissible to act upon this now?

The gist of the Fatwa:

The story mentioned by Ibn Kathir, on the authority of Ibn Ishaq, which speaks of the delegation of Najran Christians entering the Mosque and praying therein, has a problematic (Mu'dal) chain of narration, meaning it is weak. Nevertheless, even if this narration were authentic, the entry of disbelievers into the Mosque and their praying therein would be restricted to cases where there is a clear benefit, such as inviting them to Islam. Sheikh Muhammad ibn Ibrahim Al-Sheikh is of the opinion that this is permissible if there is an overriding benefit. Granting them permission to pray in the Mosque is permissible if a benefit is hoped for from it, such as encouraging them to embrace Islam and familiarizing them with worship, provided that it is not the Sacred Mosque (Masjid al-Haram).