Let’s try to answer this question using ‘Vajra-Suchika Upanishad’. It’s very short Upanishad, with a nice logical flow, so let’s go through each shloka.

1. ब्राह्मक्षत्रियवैष्यशूद्रा इति चत्वारो वर्णास्तेषां वर्णानां ब्राह्मण एव | प्रधान इति वेदवचनानुरूपं स्मृतिभिरप्युक्तम् ||
Brahmin, Kshatriya, Vaishya, Shudras are four varnas, out of them Brahmins are the chief (or guide), as per Vedic scriptures.

2. तत्र चोद्यमस्ति को वा ब्राह्मणो नाम किं जीवः किं देहः किं जातिः किं ञानं किं कर्म किं धार्मिक इति ॥
But, who is Brahmin? What decides if one is brahmin or not? Is it Body? Or Jiva (or self) inside the body? Or is it based on birth? Or knowledge? Or Karma? Or following religious duties?

3. तत्र प्रथमो जीवो ब्राह्मण इति चेत् तन्न….. No, It can’t be Jiva/Self, as it is the same Jiva that has been, taking birth as various species, animals, plants, humans, etc, in the past and will do the same in the future too. So, the Jiva/Self can’t be Brahmin.

4. तर्हि देहो ब्राह्मण इति चेत् तन्न…No, It can’t be body too, as everyone’s body is the same, be it from any varna or even outcaste, its made from the same elements. Also, bodies of people from every varna also face the same old age, vice/virtu, disease, and death.

5. तर्हि जाति ब्राह्मण इति चेत् तन्न…..No, It can’t be by-birth too, as many great Rishis have been born to non-brahmin parents. For example, Rishyasrnga, Kaushika, Jambuka, Valmiki, Vyasa, Gautama, Vasistha, Agastya none of these great rishis were born in a brahmin family.

6. तर्हि ज्ञानं ब्राह्मण इति चेत् तन्न …No, it can’t be one’s Dnyana too, as there are many mentions people from Kshatriyas and other varnas too having similar knowledge.

7. तर्हि कर्म ब्राह्मण इति चेत् तन्न …No, it can’t be karma too, as we are born to repay/reap Prarabdha karma, and current life gives rise to agami karma, which gets added to Sanchit Karma for future lives. This rule applies to everyone equally. Also, if we assign varna based on present life’s karma in ‘this life itself’, we are going against karma doctrine, by reducing the importance of past/future lives/karma.

8. तर्हि धार्मिको ब्राह्मण इति चेत् तन्न….No, it can’t be how well someone is following religious duties too, as every varna as per their abilities performs religious duties like dana, charity.

9. तर्हि को वा ब्रह्मणो नाम…Then, if none of these decides who is Brahmin, then what decides? Brahmin is one, who has attained self-realization (Brahman). Anyone having overcome emotion, spite, greed, expectation, desire, delusion, with the mind unaffected by pride, egoism, can call himself/herself a Brahmin. Only this person is eligible to be ‘chief’ of the varnas, and guide everyone on the correct path.

Surprisingly, every Indian mind, subconsciously follows this, and that is the reason, we never attach importance to which varna, Saibaba, Vivekananda, Kabir, Mirabai, and any other great saints were born. We just follow them as our Gurus.

Let’s we all too, strive to be a Brahmin, the one who has attained Brahman.

All the Best!

PS: Sanskrit Shlokas 3-9 are truncated, to reduce the post size (it’s already a very long post).