According to this formula of qualification one who does not accept Krishna as the Supreme Personality of Godhead—is unable to understand the Bhagavad-gita.
Such devotees of Krishna are actually Mahatma. Such Mahatmas are very rare. They are seen always engaged in the transcendental service of the Lord especially in the preaching work of the Lord's glories in His different energies namely the material as well as the spiritual which are described in the Bhagavad-gita as the Para and the Apara Prakrtis. These Mahatmas are always anxious to preach the universal message of peace mentioned in the Bhagavad-gita and it does not matter in whatsoever Ashram he belongs to. Arjuna was a householder Ksatriya and was fighting for the interest of the Lord and as such he was the first Acarya in the parampara line.
Arta, Artharthi, Jnani and Jijnasu are the beginners in the line of understanding the Geeta. The Mahatmas are above all these four classes of men and they are the post graduate men in the line. We know also the Ashuras who are divided into four classes namely the Duskrtinas, the Mudhas, the Naradhamas and the Foolish scholars. In order to preach the Bhagavad-gita we have to create the Mahatmas and not the Foolish Scholars who have unauthorizedly taken up the Bhagavad-gita without following the parampara line. That is an important point.