Islam literally means to submit and surrender to another's will and in the Quran, it to one who is in total submission to Allah (swt) and is free from any form of shirk and polytheism.  It is on these premises that the Quran refers to Prophet Ibrahim (pbuh) as a "Muslim".  Since the true acceptable religion to Allah (swt) is Islam[i], all of those who accepted Allah's (swt) religion in their own time [before the religion of Prophet Muhammad (pbuh)] were also Muslim, meaning that they were in total submission to Allah's will.  Nevertheless, today those who are considered Muslim are those who follow the religion of the final prophet, Prophet Muhammad (pbuh).  That is because they have proven their submission to Him by accepting His final prophet and all the previous ones as well, while followers of other religions today lack this belief, hence their kufr.  Of course, the Islam of those Muslims who don’t practice it in their everyday lives is merely a formality and the true Muslim is one who practices it and obeys all of Allah's (swt) commands; both commands that are personal and have nothing to do with others, such as prayer and fasting, and commands He has issued regarding others and their rights.