sa dvijo ’jo jagada yam
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Those who have been reformed by the garbhadhana ceremony and other prescribed reformatory methods, performed with Vedic mantras and without interruption, and who have been approved by Lord Brahma, are dvijas, or twice-born. Such brahmanas, ksatriyas and vaisyas, purified by their family traditions and by their behavior, should worship the Lord, study the Vedas and give charity. In this system, they should follow the principles of the four asramas [brahmacarya, grhastha, vanaprastha and sannyasa].
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
After giving a general list of thirty qualifications for one’s behavior, Narada Muni now describes the principles of the four varnas and four asramas. A human being must be trained in the above-mentioned thirty qualities; otherwise, he is not even a human being. Then, among such qualified persons, the varnasrama process should be introduced. In the varnasrama system, the first ceremony for purification is garbhadhana, which is performed with mantras at the time of sex for propagating a good child. One who uses sex life not for sensual pleasures but only to beget children according to the reformatory method is also accepted as a brahmacari. One should not waste semen on sensual pleasure, violating the principles of Vedic life. Restraint in sex is possible, however, only when the populace is trained in the above-mentioned thirty qualities; otherwise, it is not possible. Even if one is born in a family of dvijas, or twice-born, if they have not followed the reformatory process he is called a dvija-bandhu — not one of the twice-born, but a friend of the twice-born. The whole purpose of this system is to create good population. As stated in Bhagavad-gita, when women are polluted the populace is varna-sankara, and when the varna-sankara population increases, the situation of the entire world becomes hellish. Therefore, all the Vedic literatures strongly warn against creating varna-sankara population. When there is varna-sankara population, the people cannot be properly controlled for peace and prosperity, regardless of great legislative assemblies, parliaments and similar bodies.