Short answer
From the Islamic side, no. Wallahi means "by God", and Allah is the Arabic word for God that Arab Christians use too. Muslims are not offended when non-Muslims say wallahi or inshallah. Islam asks everyone to swear only by God and only to the truth. Whether oaths are allowed in your own church is a question for your Christian teachers, since some avoid all oaths.
For a Muslim, swearing by Allah is a valid oath (yamin), breaking an oath about the future requires kaffara (expiation), and swearing by anything other than Allah is forbidden. In the Hanafi school the oath of a non-Muslim does not bring the duty of kaffara, because kaffara is an act of worship that needs faith.
Other madhhabs
- Shafi'i
- A non-Muslim's oath is valid, and if broken the expiation by feeding or clothing the poor is due.
- Hanbali
- A non-Muslim's oath is valid and binding, with expiation if broken.
Explanation
Wallahi (or wallah) is Arabic for "by God". Arabic speakers of every faith use it. Christians in Egypt, Lebanon, Syria and Palestine say wallah, inshallah and alhamdulillah every day, and Arabic Bibles call God Allah. So a Christian friend saying wallahi is using an Arabic expression, not making a statement of Islamic faith. See wallahi meaning and inshallah meaning.
Muslims usually take it warmly when non-Muslims use these words. Allahu akbar means "God is greater", which a Christian can also mean sincerely. The one phrase to say only when you truly mean it is the shahada, since saying it with belief is how a person enters Islam.
Islam does take oaths seriously, for everyone:
- Swear only by God, never by your mother's life or anything else. The Prophet ﷺ said whoever swears should swear by Allah or stay silent.
- Swear only to the truth. A deliberately false oath is a grave sin.
- Do not add wallahi to every sentence. The Quran says "guard your oaths", and that applies to Muslims first.
Some Christians avoid every oath because of Jesus' words in Matthew 5 about letting your yes be yes, while many churches allow oaths in court and on serious occasions. That question belongs to your own tradition. From the Muslim side, a Christian who says wallahi truthfully has said nothing offensive.
The evidence
Quran, Surah Al-Maaida 5:89
لَا يُؤَاخِذُكُمُ ٱللَّهُ بِٱللَّغْوِ فِىٓ أَيْمَٰنِكُمْ وَلَٰكِن يُؤَاخِذُكُم بِمَا عَقَّدتُّمُ ٱلْأَيْمَٰنَ ۖ فَكَفَّٰرَتُهُۥٓ إِطْعَامُ عَشَرَةِ مَسَٰكِينَ مِنْ أَوْسَطِ مَا تُطْعِمُونَ أَهْلِيكُمْ أَوْ كِسْوَتُهُمْ أَوْ تَحْرِيرُ رَقَبَةٍۢ ۖ فَمَن لَّمْ يَجِدْ فَصِيَامُ ثَلَٰثَةِ أَيَّامٍۢ ۚ ذَٰلِكَ كَفَّٰرَةُ أَيْمَٰنِكُمْ إِذَا حَلَفْتُمْ ۚ وَٱحْفَظُوٓا۟ أَيْمَٰنَكُمْ ۚ كَذَٰلِكَ يُبَيِّنُ ٱللَّهُ لَكُمْ ءَايَٰتِهِۦ لَعَلَّكُمْ تَشْكُرُونَ ﴿٨٩﴾
Allah will not impose blame upon you for what is meaningless in your oaths, but He will impose blame upon you for [breaking] what you intended of oaths. So its expiation is the feeding of ten needy people from the average of that which you feed your [own] families or clothing them or the freeing of a slave. But whoever cannot find [or afford it] - then a fast of three days [is required]. That is the expiation for oaths when you have sworn. But guard your oaths. Thus does Allah make clear to you His verses that you may be grateful.
Oaths by God are serious and should be guarded, not used lightly.
Quran, Surah Al-Ankaboot 29:46
۞ وَلَا تُجَٰدِلُوٓا۟ أَهْلَ ٱلْكِتَٰبِ إِلَّا بِٱلَّتِى هِىَ أَحْسَنُ إِلَّا ٱلَّذِينَ ظَلَمُوا۟ مِنْهُمْ ۖ وَقُولُوٓا۟ ءَامَنَّا بِٱلَّذِىٓ أُنزِلَ إِلَيْنَا وَأُنزِلَ إِلَيْكُمْ وَإِلَٰهُنَا وَإِلَٰهُكُمْ وَٰحِدٌۭ وَنَحْنُ لَهُۥ مُسْلِمُونَ ﴿٤٦﴾
And do not argue with the People of the Scripture except in a way that is best, except for those who commit injustice among them, and say, "We believe in that which has been revealed to us and revealed to you. And our God and your God is one; and we are Muslims [in submission] to Him."
Muslims address Christians as people who speak of the same Creator.
Sahih al-Bukhari 2679
Sahih... من كان حالفا فليحلف بالله أو ليصمت ...
"Whoever must swear, let him swear by Allah or keep silent."
An oath may only be by God, which is exactly what wallahi says.
Where the ruling is recorded
- al-Hidaya, Kitab al-Ayman, the section on what counts as an oath and the oath of a non-Muslim. States that a non-Muslim who swears and breaks the oath owes no expiation.
- al-Mughni (Ibn Qudama), Kitab al-Ayman. The Hanbali view that a non-Muslim's oath is valid.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.