The caste system was developed long ago in India as a division of society based on the quality of a person’s personality and the type of work they do. There were four castes, or categories of people. In the Bhagvad Gita (4:13), it says:
chaturvarnam maya srushtam gunakarmavibhagashah
tasya kartaram api mam viddhy akartaram avyayam
“the system of four castes has been created by Me, based on the differentiation of qualities and actions. Although I am the creator of all these, know Me as a non-agent (the one who has not done anything- akartaram) and immutable (avyayam – without any body)
i.e. I am without caste and body and I have not done anything.
Guna means quality and karma means activity, or work. Brahmins were the first group of people and were considered to be peaceful, calm, quiet, and spiritual and their work includes study, teaching, and priestly activities. This is the simple meaning of Brahmin, but the real meaning is “Self Realised” those who are on the path of truth. Brahman janati iti brahmanah: on who knows brahman, or realizes brahman is really brahman or becomes brahman or is not different than brahman. Brahmins usually lead strict and disciplined spiritual lives, practising different religious rites and rituals and helping people to live a religious and spiritual life.
Source: page 50-51, “The Divine Quest” by Paramhansa Hariharananda of Kriya Yoga