Sh. Ibn al-ʿUthaymīn رَحِمَهُ اللهُ said, “Excommunication of a Muslim as a ruling has two prerequisites:

[1] Establishing the proof of that thing being excommunicable.

[2] Application of the ruling on one who does that, such that he is knowledgeable of it and intended it. If he was ignorant, he is not excommunicated. This is based on His, Exalted be He’s statement:

وَمَن يُشَاقِقِ ٱلرَّسُولَ مِنۢ بَعْدِ مَا تَبَيَّنَ لَهُ ٱلْهُدَىٰ وَيَتَّبِعْ غَيْرَ سَبِيلِ ٱلْمُؤْمِنِينَ نُوَلِّهِۦ مَا تَوَلَّىٰ وَنُصْلِهِۦ جَهَنَّمَ ۖ وَسَآءَتْ مَصِيرًا ۝١

{And whoever opposes the Messenger after guidance has become clear to him and follows other than the way of the believers, We shall leave him on his chosen path and burn him in Hell – what an evil destination} [al-Nisāʾ (4): 115].

And His statement:

وَمَا كَانَ ٱللَّهُ لِيُضِلَّ قَوْمًۢا بَعْدَ إِذْ هَدَىٰهُمْ حَتَّىٰ يُبَيِّنَ لَهُم مَّا يَتَّقُونَ

{And Allāh would not let a people stray after He has guided them until He makes clear to them what they should avoid. Indeed, Allāh is Knowing of all things} [al-Tawbah (9): 115].

And His statement:

وَمَا كُنَّا مُعَذِّبِينَ حَتَّىٰ نَبْعَثَ رَسُولًۭا

{And We never punish until We have sent a Messenger} [al-Isrāʾ (17): 15].

However, if he is negligent by abandoning learning and seeking clarification then he is not excused. For example: it reaches him that this action of his is disbelief, but then he does not verify it, nor research it, in that case he is not excused.

If he did not intend to do the excommunicable act, then he is not excommunicated for that. For example: if he is forced to do [an act of] disbelief while his heart is sure with faith, or his mind was overcome and he did not know what he was saying due to extreme joy or its like, like the owner of the camel which wandered off, so he lay down under a tree awaiting death, then he finds it standing before him so he takes hold of its nose-string and says ‘O Allāh, You are my slave, and I am Your Lord’ – he makes a mistake due to extreme joy.

However, whoever does an excommunicable act jokingly, he disbelieves because he intended that as the scholars have determined.”

Source: Majmūʿ Fatāwá wa Rasāʾil Faḍīlat al-Shaykh Muḥammad b. Ṣāliḥ al-ʿUthaymīn, 2: 125 – 126, no. 220.