It is true that Iman increases with acts of obedience and decreases with acts of disobedience. This is confirmed by the unanimous agreement of Ahl-ul-Sunnah wal-Jama`ah (adherents to the Sunnah and the Muslim mainstream) including the Companions (may Allah be pleased with them) and the scholars and the religious figures who followed their path. Allah, the Exalted, says,
And Allâh increases in guidance those who walk aright.
Moreover He, the Exalted, says,
The believers are only those who, when Allâh is mentioned, feel a fear in their hearts and when His Verses (this Qur’ân) are recited unto them, they (i.e. the Verses) increase their Faith
Allah, the Exalted, also says,
...and that the believers may increase in Faith (as this Qur’ân is the truth)
Regarding the decrease of Iman, Allah, the Exalted, says,
They were that day, nearer to disbelief than to Faith
The Prophet (peace be upon him) said,
He whose heart has the least faith, even as the tiniest mustard seed, will be brought out from Hell-Fire.
(Part No. 2; Page No. 426) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.