A lot of people are asked the question, if they convert to Hinduism or Sanatan Dharma what caste they would be given? Now in most cases this question is actually a rhetoric question to make a point that Hinduism divides people on caste lines. The truth is all religions, especially abrahamic religions are deeply divided based on various schools of thought. And most do not get along with each other unless they are taking on a common enemy, non-believers or Kafirs.
I will refrain from getting in to details of how they are divided and what sort of various divisions they have but will try to put some light on how Hindu Dharma has 4 divisions arranged to keep life on earth and earth itself healthy.
As per Gita chapter 4, verse 13,
चातुर्वर्ण्यं मया सृष्टं गुणकर्मविभागशः ।
तस्य कर्तारमपि मां विद्ध्यकर्तारमव्ययम् ।।
As-is translation: According to the three modes of material nature and the work associated with them, the four divisions of human society are created by Me. And although I am the creator of this system, you should know that I am yet the nondoer, being unchangeable.
Simplified explanation: According to the three qualities (goodness, passion & ignorance) of physical nature of body and the activities associated with those qualities, the four differentiation of human society is arranged.
Brahman, teacher with absolute knowledge and skills.
Kshatriya, passionate believer with skills and intelligence.
Vaisya, skilled, passionate but ignorant towards larger purpose of being a Human. Shudras, ignorant towards emotional evolution.
This division in 4 classes or caste is based on the activities needed to nurture life on earth.
Brahman, to give right knowledge to the worthy and live life for liberation at death.
Kshatriya, protect the righteous behaviour, guided by a Brahman.
Vaishyas, to trade and grow food.
Shudras, assist the actions required to accomplish activities of growing food, trading, and protection of knowledgeable people.
These classifications are a natural evolution from the nature of the principle energy from where our universe has evolved and the principle rule with which it has evolved. Since the beginning of the creation of the universe the three words Om Tat Sat were used to indicate that Om (indication of the supreme, sound that came from the black hole at the time of big bang) is the eternal principle of the absolute truth.
These three symbolic representations were used by Bahmanas, those who learned the truth and lived life practicing it to be able to liberate soul, to invoke the true self (soul) while chanting the hymns of the vedas and while behaving, so as to keep the true self (soul) satisfied and free of primal emotions of greed and fear.
At the time of birth every material body is born shudra (Ignorant), as it evolves trading unconditional love, respect with parents & teachers for food, clothes, shelter and knowledge (education) it evolves to being a Vaisya.
As an adult, to administer own life and family with passion and knowledge one evolves to a class of Kshatriya. And eventually after acquiring the knowledge and skills during the retired age one enters into being a Brahman, ready to impart the knowledge & wealth to the worthy and die with a purpose to not return on earth in any form.
The above sequence is true in case of an ideal world. Unfortunately we do not live in an ideal world today. We are in what is called a Kal Yug. Where truth is based on convenience and motivations than purpose of evolution of life to become a Human on earth.
So if someone accepts Hindu Dharma what caste that person would belong to?
IMO, the person would belong to the caste based on his/her behaviour and nature. If a person doing menial labor accepts Sanatan Dharma he will start with being a Shudra. However as he evolves to mastering skills and accepting his position in the society, the person will start evolving emotionally too. Eventually as he achieves mastery in the skills and gets complete control over his emotional state, the person in this life itself becomes a Brahman. As a Brahman if he is able to transfer his skills and knowledge to the worthy and not just to his biological children, they can and will be Brahmans too.
Being a Brahmin requires one to not just live life with knowledge but also to live life in the belief of exiting from the cycle of life and death.
In order to live life to exit from the cycle of life and death, one has to live without regrets. One can live life without regrets only when every action is based on complete consciousness free of Greed and Fear.
What is action in complete consciousness?
A person, who’s actions are perfectly controlled and conditioned by the senses and the mind, who understands actions with rationality and reasons behind those actions, gives up all sense of ownership over his wealth (physical and knowledge) and behaves for the bare necessities of life (behaves without gluttony or greed for more or fear of less).
Thus even though engaged in all sort of activities, such a person doesn’t get influenced by emotions. A person who is satisfied with the gains that come by itself while performing tasks, who is free from hypocrisy and dual nature, is not jealous, who is equipoised in success and failure, is never stuck in the confused state (state where true self/soul is in a physical body without being realised), even though going through all activities of life.
Actions of such a person who is free from the three qualities (goodness, passion and ignorance) of material nature (physical body) and who is completely living a life in absolute knowledge (knowledge of his true identity/soul) upon death achieves liberation.
To sum it up caste in Hindu Dharma is more to do with ones nature and his motivations in this life than a badge of honour or competition with belief of anyone else. Caste is not based on rituals one performs but reasons behind those rituals. Caste is not the family name one gets from his ancestors but the nature and nurture one gets from his ancestors or his teacher.
Lastly Sanatan Dharma or Hinduism is not purely a closed rules based ideology but a purpose based ideology. Which means as long as you are living life to protect and nurture life on earth without being a barrier to natural evolution you are a Dharmic. Hinduism does not see evolution only from a physical state of survival and reproduction but from a complete state of physical and emotional evolution.
Welcome to Sanatan. Welcome to Hinduism. Welcome to Hindutva.