How do you justify the caste system in Hinduism?

Swami replied:- The Gita says that the caste is based on the qualities and profession (Guna Karma Vibhagashah) of the person. [The four castes are Brahmins (priest caste), Kshatriyas (caste of warriors or noblemen), Vaishya (businessmen caste) and Shudra (laborer caste)]. Qualities were always respected. Rama and Krishna were non-Brahmins. Yet Brahmin priests wash the statues of Rama and Krishna and swallow the washings as sacred water. In the Bhagavatam it is written that Sūta, who was a low-caste person, was made Brahma in the sacrifice [in a sacrifice, the chief officiating priest is called Brahma]. Ravana was a Brahmin but was condemned. Every human being is a Shudra by birth and can become a Brahmin only by practicing the true meaning of the Vedas (Janmana Jaayate). Brahmana [Brahmin] means he who knows Brahman. Brahman also means the Veda in Sanskrit. Every caste contains good people as well as bad people. Shabari and Kannappa belonged to the family of untouchables [lower than Shudras]. Nobody is untouchable by birth. In the Veda only four castes were mentioned. How did this fifth caste come into existence? All the bad people in the four castes were expelled from the village just like a bad student is expelled from the school. Thus untouchability is due to the qualities and deeds and not by birth. Brahmins are worshipping the pictures of Shabari and Kannappa in their prayer rooms.