The Repentance - 9:99
Among the Arabs are those who believe in Allah and the Last Day. And they consider what they spend as a means to draw near to Allah and support for the messenger. Indeed it is for them a means of approach. Allah will admit them into His mercy. Allah is Forgiving, Merciful.
The most important criterion for believing in Allah:
The Cattle/6:82
Those who believe and do not mix their faith with wrongdoing, they deserve security and they have found the truth.
اَلَّذ۪ينَ اٰمَنُوا وَلَمْ يَلْبِسُٓوا ا۪يمَانَهُمْ بِظُلْمٍ اُو۬لٰٓئِكَ لَهُمُ الْاَمْنُ وَهُمْ مُهْتَدُونَ۟
The Companies/39:65
It has been revealed to you and those before you that if you associate partners, all that you do will be in vain and you will be of the losers.
وَلَقَدْ اُو۫حِيَ اِلَيْكَ وَاِلَى الَّذ۪ينَ مِنْ قَبْلِكَۚ لَئِنْ اَشْرَكْتَ لَيَحْبَطَنَّ عَمَلُكَ وَلَتَكُونَنَّ مِنَ الْخَاسِر۪ينَ
Luqman/31:13
"When Lokman advised his son, he said: "My dear son, do not associate partners with Allah; surely shirk (associating partners with God) is a great wrong."
وَاِذْ قَالَ لُقْمٰنُ لِابْنِه۪ وَهُوَ يَعِظُهُ يَا بُنَيَّ لَا تُشْرِكْ بِاللّٰهِۜ اِنَّ الشِّرْكَ لَظُلْمٌ عَظ۪يمٌ