Who is a believer (mu'min) according to the Qur'an?

ٱلَّذِينَ يُؤْمِنُونَ بِٱلْغَيْبِ وَيُقِيمُونَ ٱلصَّلَوٰةَ وَمِمَّا رَزَقْنَـٰهُمْ يُنفِقُونَ

Who believe in the Unseen, and establish worship, and spend of that We have bestowed upon them;

وَٱلَّذِينَ يُؤْمِنُونَ بِمَآ أُنزِلَ إِلَيْكَ وَمَآ أُنزِلَ مِن قَبْلِكَ وَبِٱلْـَٔاخِرَةِ هُمْ يُوقِنُونَ

And who believe in that which is revealed unto thee (Muhammad) and that which was revealed before thee, and are certain of the Hereafter.

أُو۟لَـٰٓئِكَ عَلَىٰ هُدًى مِّن رَّبِّهِمْ ۖ وَأُو۟لَـٰٓئِكَ هُمُ ٱلْمُفْلِحُونَ

These depend on guidance from their Lord. These are the successful.

۞ لَّيْسَ ٱلْبِرَّ أَن تُوَلُّوا۟ وُجُوهَكُمْ قِبَلَ ٱلْمَشْرِقِ وَٱلْمَغْرِبِ وَلَـٰكِنَّ ٱلْبِرَّ مَنْ ءَامَنَ بِٱللَّهِ وَٱلْيَوْمِ ٱلْـَٔاخِرِ وَٱلْمَلَـٰٓئِكَةِ وَٱلْكِتَـٰبِ وَٱلنَّبِيِّـۧنَ وَءَاتَى ٱلْمَالَ عَلَىٰ حُبِّهِۦ ذَوِى ٱلْقُرْبَىٰ وَٱلْيَتَـٰمَىٰ وَٱلْمَسَـٰكِينَ وَٱبْنَ ٱلسَّبِيلِ وَٱلسَّآئِلِينَ وَفِى ٱلرِّقَابِ وَأَقَامَ ٱلصَّلَوٰةَ وَءَاتَى ٱلزَّكَوٰةَ وَٱلْمُوفُونَ بِعَهْدِهِمْ إِذَا عَـٰهَدُوا۟ ۖ وَٱلصَّـٰبِرِينَ فِى ٱلْبَأْسَآءِ وَٱلضَّرَّآءِ وَحِينَ ٱلْبَأْسِ ۗ أُو۟لَـٰٓئِكَ ٱلَّذِينَ صَدَقُوا۟ ۖ وَأُو۟لَـٰٓئِكَ هُمُ ٱلْمُتَّقُونَ

It is not righteousness that ye turn your faces to the East and the West; but righteous is he who believeth in Allah and the Last Day and the angels and the Scripture and the prophets; and giveth wealth, for love of Him, to kinsfolk and to orphans and the needy and the wayfarer and to those who ask, and to set slaves free; and observeth proper worship and payeth the poor-due. And those who keep their treaty when they make one, and the patient in tribulation and adversity and time of stress. Such are they who are sincere. Such are the Allah-fearing.

إِنَّمَا ٱلْمُؤْمِنُونَ ٱلَّذِينَ إِذَا ذُكِرَ ٱللَّهُ وَجِلَتْ قُلُوبُهُمْ وَإِذَا تُلِيَتْ عَلَيْهِمْ ءَايَـٰتُهُۥ زَادَتْهُمْ إِيمَـٰنًا وَعَلَىٰ رَبِّهِمْ يَتَوَكَّلُونَ

They only are the (true) believers whose hearts feel fear when Allah is mentioned, and when His revelations are recited unto them they increase their faith, and who trust in their Lord;

قَدْ أَفْلَحَ ٱلْمُؤْمِنُونَ

Successful indeed are the believers

ٱلَّذِينَ هُمْ فِى صَلَاتِهِمْ خَـٰشِعُونَ

Who are humble in their prayers,

۞ قَالَتِ ٱلْأَعْرَابُ ءَامَنَّا ۖ قُل لَّمْ تُؤْمِنُوا۟ وَلَـٰكِن قُولُوٓا۟ أَسْلَمْنَا وَلَمَّا يَدْخُلِ ٱلْإِيمَـٰنُ فِى قُلُوبِكُمْ ۖ وَإِن تُطِيعُوا۟ ٱللَّهَ وَرَسُولَهُۥ لَا يَلِتْكُم مِّنْ أَعْمَـٰلِكُمْ شَيْـًٔا ۚ إِنَّ ٱللَّهَ غَفُورٌ رَّحِيمٌ

The wandering Arabs say: We believe. Say (unto them, O Muhammad): Ye believe not, but rather say "We submit," for the faith hath not yet entered into your hearts. Yet, if ye obey Allah and His messenger, He will not withhold from you aught of (the reward of) your deeds. Lo! Allah is Forgiving, Merciful.

إِنَّمَا ٱلْمُؤْمِنُونَ ٱلَّذِينَ ءَامَنُوا۟ بِٱللَّهِ وَرَسُولِهِۦ ثُمَّ لَمْ يَرْتَابُوا۟ وَجَـٰهَدُوا۟ بِأَمْوَٰلِهِمْ وَأَنفُسِهِمْ فِى سَبِيلِ ٱللَّهِ ۚ أُو۟لَـٰٓئِكَ هُمُ ٱلصَّـٰدِقُونَ

The (true) believers are those only who believe in Allah and His messenger and afterward doubt not, but strive with their wealth and their lives for the cause of Allah. Such are the sincere.