Bhagavad Gita 4.13चातुर्वर्ण्यं मया सृष्टं गुणकर्मविभागशः।तस्य कर्तारमपि मां विद्ध्यकर्तारमव्ययम् ॥

The four castes (Brahmana, Kshatriya, Vaishya and Sudra) are classified according to the differentiation of Guna and Karma. In a Brahmana, Sattva predominates. He possesses selfrestraint, purity, serenity, straightforwardness, devotion, etc. In a Kshatriya, Rajs predominates. He possesses prowess, splendour, firmness, dexterity, generosity and the nature of a ruler. In a Vaishya, Rajas predominates and Tamas is subordinate to Rajas. He does the duty of ploughing, protection of cattle and trade. In a Sudra Tamas predominates and Rajas is subordinate to Tamas. He does service to the other three castes. Human temperaments and tendencies vary according to the Gunas.Though the Lord is the author of the caste system, yet He is not the author as He is the nondoer. He is not subject to Samsara. Really Maya does everything. Maya is the real author. Society can exist in a flourishing state if the four castes do their duties properly. Otherwise there will be chaos, rupture and fighting. (Cf.XVIII.41).