Allah says:“By the sun and its brightness, and [by] the moon when it follows it, and [by] the day when it displays it, and [by] the night when it covers it, and [by] the sky and He who constructed it, and [by] the earth and He who spread it, and [by] the soul and He who proportioned it, and inspired it [with discernment of] its wickedness and its righteousness, He has succeeded who purifies it, and he has failed who instills it [with corruption].” (Shams, 91/1-10)

In essence, it is the inner world, emotions and thoughts that direct and shape the behavior of the person. Especially when we think in terms of morality, it will be seen that there is a will behind human behavior. This will determines whether our actions are good or bad.

The following warning of the Prophet Muhammad (saw) is very important:  "Beware! There is a piece of flesh in the body if it becomes good (reformed) the whole body becomes good but if it gets spoilt the whole body gets spoilt, and that is the heart." (Bukhari, Iman, 39; Muslim, Musaqat, 107) “The container leaks out whatever is inside” in accordance with this axiom, the better the inner world of a human, the better his behavior will be. Prophet Muhammad (saw) points out the importance of the heart and conscience by saying “If a person feels joy when doing good and sorrow when doing something bad, than that person is a believer” (Ibn Hanbal, IV, 399).

A believer can't say “After all, nobody knows what I think, what I plan for, nor what I truly feel in my heart" and have certain ill-intended thoughts. A believers does not forget that Allah knows the feelings and thoughts that pass through one's heart. He also knows that bad feelings such as grudge, hatred and envy that are fed in the heart will reflect on the behavior of the person over time and lead the person to evil. On the other hand, the good feelings and thoughts of the heart will ensure that both a person's inner world and his behaviors become good. Even good and bad deeds are valued according to the intention of the person. The following hadith of the Prophet Muhammad (saw) states that the most important thing is intention, emotion and thought: "The reward of deeds depends upon the intentions."(Bukhari, Bad'u al-wahy, 1) Accordingly, even if a believer wants to do good things but cannot do them, he will be rewarded by Allah (swt) for this good intention. The same is true for the hypocritical person who shows off. If a person does things like praying, fasting, and helping people, which are seen as good deeds from an external point of view, with a heavy heart and not to win the love of Allah, but because he is afraid of the reaction of the people around him or to show off, what he does will not have a value in the presence of Allah. 

Allah says "The Day when there will not benefit [anyone] wealth or children, but only one who comes to Allah with a sound heart." (Shu'ara, 26/88-89) , this indicates the value of a goodwill in the eyes of Allah. So believers should act with the awareness that all believers are brothers, and strive to love each other with sincere feelings, help each other, share each other's troubles and think well about others, and to be forgiving, smiling and sweet-spoken, avoid prejudices, bad feelings and thoughts. In short, a believer should know how to control his negative feelings and thoughts. 

The Prophet Muhammad (saw) emphasized that the essential thing in religion is sincerity with the following hadith: “Verily Allah does not look to your faces and your wealth but He looks to your heart and to your deeds.” (Muslim, Birr, 34) It should not be forgotten that “Allah does not accept any deed, except that which is purely for Him, and seeking His Face.” (Nasa'i, Jihad, 24)