Can a Muslim trust the sincerity of an unbeliever's oath sworn by other than Allah, such as Christians swearing by Jesus, peace be upon him?

It is not permissible to swear by anyone other than Allah Almighty, whether one is a Muslim or a disbeliever. The testimony of disbelievers against Muslims is not permissible except in the case of a will made during travel when there are no believers present. An oath sworn by a disbeliever is valid if he swears by Allah, and expiation becomes binding upon him if he breaks it. However, if he swears by other than Allah, his oath is not valid and does not take effect. The evidence for the validity of a disbeliever's oath if he swears by Allah is the Hadith of Al-Ash'ath ibn Qays.