What is the picture of a believer in the Quran? Who is a believer in the Quran?
The criterion is the Quran. If we set a criterion on our own and assume we are believers, then this is wishful thinking. A person who measures up to the criterion set by God will be a believer in God's eyes. If you have a lot of money, you become happy with the wealth. You go to the bank to store this money and have a bank balance. But when you present the notes to the bank clerk,  he informs you that the notes are fake. These notes have no value. Suddenly, your dreams will come down crashing. Your wishful thinking will come to an end. Those notes are really notes which are certified by the bank as true. Similarly, only that faith is true faith which is certified by God as such. Otherwise, it will be a fake faith, which is valueless. If a person wants himself to be considered a believer in the eyes of God, then he must judge himself according to the criterion given in the Quran.  The Quran says: Copy Arabic text (3:110)

You are indeed the best community that has ever been brought forth for [the good of] mankind.  Hazarat Umar has said: Copy Arabic text (Tafsir Ibn Kathir 2:103)

If a person wants God to make him member of khayr-i ummat, then he must fulfil the conditions required for receiving the rewards of khayr-i ummat. If a person to be member of khayr-i ummat, then he must fulfil certain conditions. Otherwise, he will not be regarded as part of khayr-i ummat. If a person who wants to live and die as a believer, he must read the Quran.  He must discover from the Quran the criterion of becoming a believer and mould his life according to it.