A note on locking mosques

Answer: It is obligatory to protect the mosque’s property; it is not permissible to expose it to thieves or to be negligent regarding it. There is no objection—so far as appears to me—to locking the mosque for that reason outside the customary prayer times. As for during them, it is not permissible, due to His saying, Exalted is He: “And who are more unjust than those who prevent the name of Allah from being mentioned in His mosques and strive toward their ruin.” [Al-Baqarah:114]
If it be said: The apparent sense of the verse indicates that locking mosques is not permissible at all—neither in prayer times nor outside them—
we say: Locking the mosque after the first third of the night until pre-dawn is not a “prevention,” for no one generally frequents mosques at that time; thus it is not a prevention of the remembrance of Allah therein, but rather a prevention of thieves taking the mosque’s property. If, on rare occasion, someone comes to pray at that time and finds it locked, there is no blame upon the overseer for locking it, since he did not intend to prevent and did not will it. Allah, Exalted is He, has said: “There is no blame upon you for what you did by mistake, but [only] for what your hearts intended.” [Al-Aḥzāb:5]
Yes: if the mosque is well-funded and worshipers do not cease frequenting it—like the Mosque of Imām al-Hādī (peace be upon him) in Ṣaʿdah—then the overseer must open it as need dictates and arrange for a guard to protect it from thieves. This is what appears in this matter; it reflects due care to safeguard the mosque’s property and to serve those who frequent the mosques—to the extent possible.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1