In plain words
Or is it not thinking 'we know everything' -- what else is that but ego? Ego too is nothing but a progeny of ignorance, isn't it! Based on this, it is beyond doubt that lineage, sect, caste and the like arise on the basis of guṇa and karma. It is essential to recognize the truth that these are not man-made divisions. For these, the Veda alone is the authority; the Veda alone is the basis.
The land of Bhārata is called 'the land of karma'. This is the field of karma; every human being is a pilgrim to this field of karma, and that is why karma is paramount in this country. For that very reason this very name came to be given to it. High and low birth, fortune and enjoyment, bondage and Mokṣa -- all lie in our own hands, so the Śruti proclaims, raising its voice. Saying what? 'sayadhākratu rasmin loke puruṣo bhavati tadhetaḥ pratye bhavati.' Its meaning is this: whatever form a man prays for in this world, that same form is attained again after death. That birth arises in accordance with karma is the truth. Our present enjoyment and fortune, our natures, qualities, intellects, and pleasure and pain, are all things earned in a previous birth.
In just the same way, the next birth is shaped according to whatever dispositions we follow in performing our deeds in this birth. Some people are born into a good lineage and yet practice bad deeds; others are born into a low lineage and yet perform good deeds. We commonly discuss among ourselves what the truth of this is. That is: some, born into the brāhmaṇa caste, practice bad deeds -- that is, deeds of rajo- and tamo-guṇa. Others, born into a lower lineage, practice sattvic, good deeds. Such people are brāhmaṇas by birth only, but they are not brāhmaṇas by deed; they are low by birth alone, but not low by deed. The Veda too accepts that it is only those persons in whom birth and deed both coincide who are the true proof of a correct lineage.