Without becoming dvija, one cannot understand this Vedic literature. Just like without becoming a graduate, you are not allowed to enter in the law court. Similarly, without becoming a dvija, you are not allowed to study the Vedas, because you will not understand. Saṁskārād bhaved dvijaḥ veda-pathād bhaved vipro. And when by studying the Vedic literature he understands Kṛṣṇa, then he becomes a Brahmana. Vedaiś ca sarvair aham eva vedyam (BG 15.15).
Because the purpose of studying Vedas means to understand Kṛṣṇa, or God. When I speak of Kṛṣṇa, you understand God. So when one understands what is God, what is Brahman, what is Kṛṣṇa, what is Paramātmā, then he is a Brahmana. This is the process. So the Garga Upaniṣad says that anyone who dies after becoming a Brahmana, his life is successful. And anyone who dies without becoming a Brahmana, he dies like a cat and dog. That is the difference.