Answered by Shaykh ‘Abdur-Rahmaan ibn Naasir As-Sa’dee – rahimahullaah.
This is a great question indeed. By knowing the difference between the believers and other people, the difference between the truth and falsehood becomes distinctly clear, as does the difference between the people of eternal happiness and the people of eternal misery.
Know that the true believer is the one who believes in Allaah and all of his Names and Attributes that are found in the Book and the Sunnah, upon their proper understandings. They must be fully acknowledged. Furthermore, the believer exalts Allaah, declaring Him free of anything that contradicts the perfection of His Names and Attributes. Thus, his heart becomes full of Eemaan, knowledge, conviction, serenity, and becomes totally concerned with Allaah.
The believer repents to Allaah alone, performing acts of worship for Him that He has legislated upon the tongue of his Prophet (sallallaahu ‘alayhe wa sallam), with sincerity to Allaah, hoping for His Reward and fearing His Punishment.
The believer is thankful to Allaah in his heart, with his tongue, and with the actions of his limbs. He is thankful for Allaah’s Blessings and the great amount of goodness that Allaah has given him, those that he enjoys at all hours of the day. The believer is thankful and utters phrases of Allaah’s remembrance, and he does not see any blessing greater than what Allaah has given him, nor any token of generosity greater than what has come from Allaah.
The believer scoffs at the materialistic fancies of this worldly life, as he is focused on the delight of repenting to Allaah, and turning completely to Him alone. With this, he takes an ample share of worldly delights and enjoys them, but not as the disbelievers or heedless ones do. He indulges in them in a way that assists him in establishing the Rights of Allaah and the rights of His servants.
By contenting himself in this way and hoping for the reward from Allaah, the believer fulfills his desires, his heart rests peacefully, and he finds tranquility. He does not become saddened if some affairs reach him that he does not like. With this, Allaah has combined for him the happiness of this worldly life and the happiness of the Hereafter.
As for the heedless disbeliever, then his affair is not like this. He denies the presence and perfection of His Great Lord, that which is established by way of both intellectual and textual proofs, as well as scientific facts and findings. He does not pay any mind to these things.
As he cuts himself off from Allaah by not acknowledging Him and not worshipping Him, the disbeliever attaches his self to things found in nature and worships them. Thus, his heart becomes similar to the hearts of grazing animals.
The disbeliever has no concern other than gathering and enjoying material things. His heart is always in a state of unrest, afraid that he may lose the things he loves, afraid that affairs he does not like will reach him. He does not have the Eemaan that makes facing difficult things easy and lightens the weight of calamities.
The disbeliever has been deprived of the delight of Eemaan, the sweetness of drawing close to Allaah, and the fruits of Eemaan in this life and the Next.
The disbeliever does not hope for any reward from Allaah, nor does he fear any punishment. His only hope and fear is connected to the cravings of his lowly, materialistic, worldly desires.
From the characteristics of the believer is humbleness when submitting to the Truth, and humbleness when dealing with others as well. He is sincere to the servants of Allaah no matter what their status or position. This sincerity is found in his statements, actions, and intentions.
However, the disbeliever has pride and looks down upon the Truth. He looks down upon other people, too. He is amazed at himself, and does not make a practice of being sincere to anyone.
The believer has a clean heart that is free of deception, rancor, and jealousy. He loves for the Muslims what he loves for his own self, and he hates for them what he hates for his own self. He dedicates himself to working in their best interests. He carries the burden of being harmed by them, and does not oppress them in any way.
The disbeliever’s heart is boiling with rancor and jealousy. He does not want any good or benefit for anyone else, unless there is some worldly goal in it for his own self. He does not mind oppressing others when he is able, while he is the weakest of them in bearing their harms that reach him.
The believer has a truthful tongue and interacts with people in a good manner. He is gentle, calm, peaceful, merciful, patient, and trustworthy. He is easy to be around and a good host to his guests.
The disbeliever is rash, harsh, and full of worries and dismay. He lies and is not trustworthy. He is quarrelsome with an aggressive temperament.
The believer does not render himself completely to other than Allaah. Both his heart and face have become safeguarded against submitting fully or rendering themselves to other than their Lord. He is virtuous, strong, brave, and generous. He has integrity, and chooses what is good.
The disbeliever is upon the opposite of that. His heart is attached to the created beings. He fears their harm and hopes to benefit from them, expending all of his energies for them. He is not virtuous, nor is he strong or brave, unless it is to achieve his lowly motives. He has no integrity, nor does he care if what he earns is pure or filthy.
The believer combines seeking after the means that benefit him and placing his trust in Allaah, relying on Him, and seeking help from Him in all his affairs. He finds that Allaah the Most High helps him.
The disbeliever knows nothing of tawakkul, nor does he look to other than his own weak, humiliated self. Allaah has given him abilities that he uses only to benefit his self, and thus caused them to become useless. If he attains some things that he loves, then he is only being led into a gradual punishment.
The believer meets all blessings that come his way with thankfulness, and he uses them to benefit himself in a way that will cause more good to come.
The disbeliever meets blessings with exuberance and pride, busying himself with the blessing rather than the One who gave it to him, and thus does not offer any thanks. He uses the blessings for his lowly goals. With all his exuberance, his blessings are quick to part from him.
The believer meets calamities with patience and counts on Allaah’s Reward, hoping for Allaah to remove the difficulty. The goodness and the reward that Allaah gives him in place of his loss is greater than the beloved thing he lost or the hardship he endured.
The disbeliever meets calamities with dismay and worry, and thus his calamity increases and he is hurt both outwardly and within his heart. He has no patience or hope for any reward. How severe is his loss and how great is his sadness!
The believer worships Allaah by having faith in all of the Messengers. He honors them and loves them more than he loves the rest of the entire Creation. He acknowledges that all goodness that reaches the Creation until the Day of Standing is only the result of their work and by their guidance. He knows that any evil that reaches them is a result of their opposition to the Messengers. He further knows that they are the greatest of all the Creation, in terms of the goodness they spread, especially their imaam and final seal, Muhammad (sallallaahu ‘alayhe wa sallam), the one that Allaah made as a mercy to all the Creation and sent him with every kind of righteousness, to correct and guide the people.
The criminal disbelievers are upon the opposite of that. They sanctify the enemies of the Messengers and honor their statements. They mock what the Messengers came with, just as their predecessors did. This is the biggest evidence of the absurdity of their intellects and the extreme lowliness of their manners.
The believer worships Allaah by loving the Companions and the imaams of the Muslims, the imaams of guidance.
The disbeliever is upon the opposite of that.
The believer performs deeds for the sake of Allaah and behaves well with the servants of Allaah, due to his complete sincerity to Allaah.
The disbeliever only works to achieve his own lowly goals.
The believer’s chest is expanded, full of beneficial knowledge, sound Eemaan, turning to Allaah, remembering Him, and kindness to others. His chest is free of lowly characteristics.
The heedless disbeliever is upon the opposite of that, since he has not taken any measures to expand his chest in such a way.
Source: Essential Questions and Answers Concerning The Foundations of Eemaan & Obstacles In the Path of Eemaan, Two Treatises by the ‘Allaamah ‘Abdur-Rahmaan ibn Naasir As-Sa’dee (d.1376), T.R.O.I.D. Publications, pp.58-63.