To elaborate the above question, can a person become a Muslim without believing in Islam but only verbally testifying a simple Shahadah like below, which he does believe in without going into the details of it :
أشهد أن لا إله إلا الله و أسهد أن محمد رسول الله I witness that there is no god but Allah and Muhammad is His Messenger
I have come across many people who wont mind witnessing the above fact , they would also accept Muhammad pbuh as Messenger of God, but would continue following their old ways\religion. Which means Imaan has not entered their hearts.
Moreover I remember a Sahih hadith wherein the prophet approved the Shahadah of a person who apparently said it out of fear to a Sahabah to be saved from getting killed in a War.
From what I read in the historical sources I also found a similar phenomenon , wherin people would embrace Islam en Mass without Imaan even entering into their hearts. Their entering into Islam was just a step to start the training to develop Imaan as evidenced from the following verse:
The bedouins say: "We believe." Say: "You believe not but you only say, 'We have surrendered (in Islam),' for Faith has not yet entered your hearts. But if you obey Allah and His Messenger (SAW), He will not decrease anything in reward for your deeds. Verily, Allah is Oft-Forgiving, Most Merciful." (49:14)
This means that there were people at the time of the prophet pbuh who were Muslims but were not Mumeenineen(Belief entering their hearts). So does it mean one can be a Muslim without Imaan?