Belief in Allah (swt) means that a person submits to Him in what He commands and forbids, and does not take any lord or deity besides Him (swt).
(… وَهُوَ مُحْسِنٌ …) (…while being a doer of good …): It is not sufficient to pray, fast, and perform acts of worship without doing a good (Ihsan), and Ihsan is to worship Allah (swt) as if you see Him and if you do not see Him, then He sees you, and it is in everything and above everything:
(and to parents do good) (An-Nisaa: 36)
This is the religion of Islam, the religion of benevolence to parents, relatives, neighbors, society, all people, birds, animals and plants.
وَمَنْ أَحْسَنُ قَوْلًا مِّمَّن دَعَا إِلَى اللَّهِ وَعَمِلَ صَالِحًا وَقَالَ إِنَّنِي مِنَ الْمُسْلِمِينَ % وَلَا تَسْتَوِي الْحَسَنَةُ وَلَا السَّيِّئَةُ ۚ ادْفَعْ بِالَّتِي هِيَ أَحْسَنُ فَإِذَا الَّذِي بَيْنَكَ وَبَيْنَهُ عَدَاوَةٌ كَأَنَّهُ وَلِيٌّ حَمِيمٌ % وَمَا يُلَقَّاهَا إِلَّا الَّذِينَ صَبَرُوا وَمَا يُلَقَّاهَا إِلَّا ذُو حَظٍّ عَظِيمٍ
(And who is better in speech than one who invites to Allah and does righteousness and says, “Indeed, I am of the Muslims.” % And not equal are the good deed and the bad. Repel [evil] by that [deed] which is better; and thereupon the one whom between you and him is enmity [will become] as though he was a devoted friend. % But none is granted it except those who are patient, and none is granted it except one having a great portion [of good]) (Fussilat: 33-35)
( وَاتَّبَعَ مِلَّةَ إِبْرَاهِيمَ حَنِيفًا ۗ وَاتَّخَذَ اللَّهُ إِبْرَاهِيمَ خَلِيلًا … ) (…and follows the religion of Abraham, inclining toward truth? And Allah took Abraham as an intimate friend): The prophets (PBUT) came from Ibrahim, Isaac and Ismail, and from Isaac came Jacob, and from Jacob came the tribes and Joseph, and after them came Moses, Jesus, David, Solomon, Zakaria, Yahya and Ismail, from whom came the Prophet Muhammad (saws).
Allah (swt) made Ibrahin (PBUH) a Khalil (as an intimate friend) because of the many trials he faced, of which the first is:
وَإِذِ ابْتَلَىٰ إِبْرَاهِيمَ رَبُّهُ بِكَلِمَاتٍ فَأَتَمَّهُنَّ ۖ قَالَ إِنِّي جَاعِلُكَ لِلنَّاسِ إِمَامًا ۖ قَالَ وَمِن ذُرِّيَّتِي ۖ قَالَ لَا يَنَالُ عَهْدِي الظَّالِمِينَ
(And [mention, O Muhammad], when Abraham was tried by his Lord with commands and he fulfilled them. [Allah] said, “Indeed, I will make you a leader for the people.” [Abraham] said, “And of my descendants?” [Allah] said, “My covenant does not include the wrongdoers.”) (Al-Baqara: 124)
Another trial when Nimrod threw him (PBUH) in fire, at that moment, Ibrahim was visited by the Angel Jibreel, who said: “Is there anything you wish for?”. Ibrahim (PBUH) answered: I have no request from you. As for Allah, it is enough for Him to know of my condition”. This certainty and deep faith from the father of the Prophets, Ibrahim Al-Khalil, made him assume this position, Khalil Al-Rahman (As an intimate friend of Allah), as well as the affliction with regard to Hajar when he said:
رَّبَّنَا إِنِّي أَسْكَنتُ مِن ذُرِّيَّتِي بِوَادٍ غَيْرِ ذِي زَرْعٍ عِندَ بَيْتِكَ الْمُحَرَّمِ رَبَّنَا لِيُقِيمُوا الصَّلَاةَ فَاجْعَلْ أَفْئِدَةً مِّنَ النَّاسِ تَهْوِي إِلَيْهِمْ وَارْزُقْهُم مِّنَ الثَّمَرَاتِ لَعَلَّهُمْ يَشْكُرُونَ
(Our Lord, I have settled some of my descendants in an uncultivated valley near Your sacred House, our Lord, that they may establish prayer. So make hearts among the people incline toward them and provide for them from the fruits that they might be grateful.) (Ibrahim: 37)
Where Allah (swt) commanded him to leave her and her infant son Ismail in a valley in which there was no plant, animal, bird, or water at that time, so he complied with the command of Allah (swt), and then Hajar said: “Allah will not forsake us”. After that, Allah (swt) tried him with his son, Ismail (PBUH):
فَلَمَّا بَلَغَ مَعَهُ السَّعْيَ قَالَ يَا بُنَيَّ إِنِّي أَرَىٰ فِي الْمَنَامِ أَنِّي أَذْبَحُكَ فَانظُرْ مَاذَا تَرَىٰ ۚ قَالَ يَا أَبَتِ افْعَلْ مَا تُؤْمَرُ ۖ سَتَجِدُنِي إِن شَاءَ اللَّهُ مِنَ الصَّابِرِينَ
(And when he reached with him [the age of] exertion, he said, “O my son, indeed I have seen in a dream that I [must] sacrifice you, so see what you think.” He said, “O my father, do as you are commanded. You will find me, if Allah wills, of the steadfast.” ) (As-saffat: 102)