All societies have some sort of social class system in which people are classified based on education, culture, and income levels. The classification was wrongly called as the caste system.
1)The priestly/intellectual class
2)The warrior class or kshatriyas
3)The trade/commerce class or vaishyas
4)The agricultural/labor class or shudras
Castes or jatis meaning birth, are are complex social groups to fit in to any model. Varna, is a theoretical model and like Aryan and Dravidian theorey, these are very over simplification of reality. The four varnas can not represent complex systems of thousands of castes. The term Castes, means different things to different Indians. You can not map castes to varnas, as one caste may have people in more than one varna. Varna system had not been rigid, even during Ramayana and Mahabharatha period.
These divisions are more to do with Indian society rather than any religion. So Indian converts to other religion still live with the caste system. Divisions like clans, race exist in all societies in the world, irrespective of religion they follow.
The Four Varna system of ancient India was originally based upon the idea of a social order. Evolution in society depends upon the differentiation of roles and professions. Older societies worldwide, including Europe up through the nineteenth century, had similar social orders of priests, aristocracy, merchants and common people, remnants of which can be found in many countries today.
The biggest problem for all social orders is determining the place and aptitude of each individual within it. One’s family of birth can be an important factor for determining the profession a person naturally belongs to. General perception, "it is more likely that a good musician will arise from a family of musicians, where he can be trained from early childhood, rather than from a non-musical family". There will be many exceptions to this rule, as the child of a great musician may not be good at music at all.
For creating a society that honors the aptitude, abilities and contribution of each individual, diversity needs to be explained properly for people to understand.