Allah supports His Prophet
Allah said,
إِلاَّ تَنصُرُوهُ
(If you help him not), if you do not support His Prophet , then it does not matter, for Allah will help, support, suffice and protect him, just as He did,
إِذْ أَخْرَجَهُ الَّذِينَ كَفَرُواْ ثَانِيَ اثْنَيْنِ
(when the disbelievers drove him out, the second of the two;) During the year of the Hijrah, the idolators tried to kill, imprison or expel the Prophet , who escaped with his friend and Companion, Abu Bakr bin Abi Quhafah, to the cave of Thawr. They remained in the cave for three days so that the pagans who were sent in their pursuit, returned (to Makkah), and they proceed to Al-Madinah. While in the cave, Abu Bakr was afraid the pagans might discover them for fear that some harm might touch the Messenger ﷺ. The Prophet kept reassuring him and strengthening his resolve, saying,
«يَا أَبَا بَكْرٍ، مَا ظَنُكَ بِاثْنَينِ اللهُ ثَالِثُهُمَا»
(O Abu Bakr! What do you think about two, with Allah as their third) Imam Ahmad recorded from Anas that Abu Bakr said to him, "I said to the Prophet when we were in the cave, `If any of them looks down at his feet, he will see us.' He said,
«يَا أَبَا بَكْرٍ، مَا ظَنُكَ بِاثْنَينِ اللهُ ثَالِثُهُمَا»
(O Abu Bakr! What do you think about two with Allah as their third)" This is recorded in the Two Sahihs. This is why Allah said,
فَأَنزَلَ اللَّهُ سَكِينَتَهُ عَلَيْهِ
(Then Allah sent down His Sakinah upon him) sent His aid and triumph to His Messenger , or they say it refers to Abu Bakr,
وَأَيَّدَهُ بِجُنُودٍ لَّمْ تَرَوْهَا
(and strengthened him with forces which you saw not), the angels,
وَجَعَلَ كَلِمَةَ الَّذِينَ كَفَرُواْ السُّفْلَى وَكَلِمَةُ اللَّهِ هِىَ الْعُلْيَا
(and made the word of those who disbelieved the lowermost, while the Word of Allah that became the uppermost;) Ibn `Abbas commented, "'The word of those who disbelieved', is Shirk, while, `The Word of Allah' is `La ilaha illallah." It is recorded in the Two Sahihs that Abu Musa Al-Ash`ari said, "The Messenger of Allah ﷺ was asked about a man who fights because of courage, or out of rage for his honor, or to show off. Whom among them is in the cause of Allah' The Prophet said,
«مَنْ قَاتَلَ لِتَكُونَ كَلِمَةُ اللهِ هِيَ الْعُلْيَا فَهُوَ فِي سَبِيلِ الله»
(He who fights so that Allah's Word is superior, then he fights in Allah's cause.)" Allah said next,
وَاللَّهُ عَزِيزٌ
(and Allah is All-Mighty), in His revenge and taking retribution, He is the Most Formidable and those who seek refuge with Him and take shelter by adhering to what He instructs are never made to suffer injustice,
حَكِيمٌ
(All-Wise), in His statements and actions.
Tafsir Ibn Kathir (abridged, English).