Islam is universal in outlook, it is non-dogmatic, non-doctrinal, and it is simply the submission to the Will of Allah (SwT) that is needed to stay in the realm of Islam. It entails having faith, doing right and eschewing wrong as the Holy Verse lies down:
''You are the best of peoples, evolved for mankind, enjoining what is right, forbidding what is wrong, and believing in Allah. If only the People of Book had faith, it were best for them; among them are some who have faith, but most of them are perverted transgressors [4:110]
The Holy Verse relates to historical evolution of religions which brings forth the 'Universal Truth' the common denominator of People to whom Book was revealed called 'Ahl-e-Kitab'. The universality of 'Truth' called 'HAQ' in the Islamic realm is an attribute of Allah (SwT). To this 'Truth' we may submit by holding on to 'Faith' doing right and eschewing wrong.
Doing right is not an objective related to personal sphere only; it also implies setting an example for others to do right, as also gathering the strength of character to see that right prevails. On a similar plane eschewing wrong entails being an example for others to eschew wrong, and take all care to see that wrong is defeated.
'Universal Truth' was revealed as the religions evolved, it did not go beyond the common denominator, asking to hold on to 'Faith' steadfastly, do right and eschew wrong. Yet 'People of the Book' barring a few transgressed the norms set for submitting to the Will of Allah (SwT) and in doing so they moved beyond the common denominator. Instead of non-dogmatic and non-doctrinal approach, dogmatism followed, one doctrine followed another, resulting in endless harm. There is a price to be paid for transgressing norms, we witness it, yet take no lessons.