The deen of islam and what is islam.

 The deen of islam and what is islam. 

Islam means submission from the root سلم=slim meaning submit. 

Allah s.w.t says in Quran, 

Al-Baqarah 2:208

O you who have believed, enter into Islām completely [and perfectly] and do not follow the footsteps of Satan. Indeed, he is to you a clear enemy.

Entering islam is to surrender to Allah. 

With worshipping him alone(tawheed) and being sincere to him in worship

That is islam. 

And one cannot enter islam without declaration of faith which is laa ilaha illa Allah, muhammadur rasuulullaah.

That is to say there is none worthy of worship except Allah and muhammad is his messenger. 

This is what makes one a muslim

We proclaim by the tongue as the heart confirms and action acts upon it. 

This is the true declaration of faith. 

Many muslims utter this kalimatayn yet associate partners with Allah and going to soothsayers and when something happens to them alledged it on something or someone. 

This is kufr and takes one out of islam. 

As Allah says, 

Yusuf 12:106

وَمَا يُؤْمِنُ أَكْثَرُهُم بِٱللَّهِ إِلَّا وَهُم مُّشْرِكُونَ 

And most of them believe not in Allāh except while they associate others with Him.