The eeman and the basis of it.

 The eeman and the basis of it. 

Alhamdulillah, Faith means Eeman and it increases and decreases. 

Allah says in Quran, 

Aal-e-Imran 3:173

Those to whom people [i.e., hypocrites] said, "Indeed, the people have gathered against you, so fear them." But it [merely] increased them in faith, and they said, "Sufficient for us is Allāh, and [He is] the best Disposer of affairs."[1]

Allah says, 

Al-Anfal 8:2

The believers are only those who, when Allāh is mentioned, their hearts become fearful, and when His verses are recited to them, it increases them in faith; and upon their Lord they rely -

The Eeman(Faith) increases as one does good deeds and decreases when one does evil and bad deeds. 

As reported in hadith, 

It was narrated from Abu Hurairah that the Messenger of Allah (ﷺ) said:

When the believer commits sin, a black spot appears on his heart. If he repents and gives up that sinand seeks forgiveness, his heart will be polished. But if (the sin) increases, (the black spot) increases. That is the Ran that Allah mentions in His Book: “Nay! But on their hearts is the Ran (covering of sins and evil deeds) which they used to earn.” [83:14]

Sunan ibn Majah. 

Vol. 5, Book 37, Hadith 4244.

And there are more than sixty times of faith. The greatest is to proclaim there is no God except Allah and the least is to remove pebbles on the pathway. 

As we know, 

It is obligatory upon every muslim to believe in the six articles of faith. 

Believe in Allah, his Angels, his sacred books, his messengers, believe in the last day and believe in Qadr(destiny whether good or evil is from Allah).

And anyone who disbelieve in one of these article of faith becomes a disbeliever.

Allah says, 

Al-Baqarah 2:8

And of the people are some who say, "We believe in Allāh and the Last Day," but they are not believers.