There is no constraint in Islam

Islam invites people to accept Islam without pressurising them. The system of Jihaad in Islam is not used to force people to accept Islam. This was done to create peace in the entire world because all forms of Kufr are a cause of فساد (disorder / corruption) on earth. And Jihaad was instituted to combat فساد in all its forms, so that humanity could live in peace and freedom with honour and dignity, in the obedience of the One Supreme Creator - "Allah Subhanahu Wa Ta’ala". 

When we ponder also, we see that a person cannot be forced to accept Islam because a Muslim is someone who has Imaan in his heart. One can put pressure on the body of a person to make him do something forcefully. But one will not force him to believe or accept something that he does not want. Thus, it is impossible to force a person to accept Islam. 

But...

But as soon as a person embraced Islam, it is not authorised in any way to renounce Islam. Now, he does not have the right to convert to another religion.

[English version of "Perna aukene contrainte dans l’Islam" – Al Jamiat – Vol.4 No.8]