السَّلَامُ عَلَيْكُمْ

Some people say that once you believe in the necessary beliefs you need as a Muslim, you are complete as a Muslim. However this isn’t the case. Rather you putting those core beliefs in your heart is the core, and your body has to go into line from that core. The evidence for this is the Hadith in Sahih Bukhari 52 (1):

The Prophet ﷺ said,
“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.”

So if you put the right beliefs in your heart, put sincerity in your heart, and the following of the Prophet ﷺ in your heart, the rest of the body will be good because your heart is good.

This is unlike those who say, “Imam is only in the heart and it doesn’t occur in the limbs.” This is incorrect, and this is the belief of the deviant sect called the Murjiah.

The Murjiah don’t have the correct Aqidah. They believe that Iman isn’t in the limbs. They also believe that either you have Iman or you don’t. All of this is not in line with the Aqidah of AhlusSunnah.

The Aqidah of AhlusSunnah is that Iman goes up and down and that it shows on your limbs. In fact actions are in of themselves Iman.

Allah says,
“And never would Allāh have caused you to lose your faith [i.e., your previous prayers]. (2:143)“

Here the action of prayer is considered Iman. So prayer is Iman and it increases you in Iman.

Allah knows best.

Sources:

Sahih al-Bukhari 52:
https://sunnah.com/bukhari:52