Birth progression

Source: https://twitter.com/GhorAngirasa/status/1338885970788114434

“He who has purified his soul by good deeds and controlled his senses; such a shUdra should be attended upon with respect like a dvija; thus is the dictate of brahma”

ब्राह्मण्यं देवि दुष्प्रापं
निसर्गाद् ब्राह्मणः शुभे ।
क्षत्रियो वैश्य-शूद्रौ वा
निसर्गाद् इति मे मतिः ॥६॥

brAhmaNyam (brAhmaNa-ness) is difficult to obtain; it comes innately; so does kSatriyatvam, etc.

त्रेताग्नि-मन्त्र-विहितो
वैश्यो भवति वै यदि ॥ स वैश्यः क्षत्रियकुले
शुचौ महति जायते ॥३३॥

After listing all the acts a vaishya should do, including the maintenance of 3 shrauta fires, mahAdeva tells umA that such a vaishya will take birth (jAyate) in a kSatriya family.

गोब्राह्मण-हितार्थाय
रणे चाभिमुखो हतः ।
त्रेताग्नि-मन्त्र-पूतं वा
समाविश्य द्विजो भवेत् ॥४३॥ ज्ञानविज्ञान-सम्पन्नः
संस्कृतो वेदपारगः ।
विप्रो भवति धर्मात्मा
क्षत्रियः स्वेन कर्मणा ॥४४॥

The kSatriya who goes over & above the duties meant for him: who is forward-facing in battle for the welfare of cows & brAhmaNas and he who enters his three shrauta fires [after death]; such a kSatriya becomes a dvija (means brAhmaNa here);

Becomes? In this life? So, a kSatriya can just become a brAhmaNa then? No, because the shloka clearly refers to one who was slain (hataH) for the noble ends and/or reached the other world after being committed to his three holy fires in the cremation (samAvishya). Thus, no doubt. No doubt that this is referring to a subsequent birth. To ensure that dvija indeed refers to brAhmaNa, the next shloka says, “vipra”. Such a dharmAtmA kSatriya will become a brAhmaNa. in his next birth.

एते योनि-फला देवि
स्थान-भाग-निदर्शकाः ।

The division based on birth from certain wombs as being the division of the states souls obtain as the fruits of their deeds (i.e. birth-based varNas exist as a mechanism for facilitating karmic rewards).

एतैः कर्मफलैर् देवि
न्यूनजाति-कुलोद्भवः ।
शूद्रोऽप्य् आगम-सम्पन्नो
द्विजो भवति संस्कृतः ॥४५॥

mahAdeva concludes his discourse to umA by saying, “Through these acts, one born in a family that is deficient [of the vedic rites] becomes one endowed with scriptural revelation (Agama-sampanna), a refined dvija he becomes.” …
कर्मभिः शुचिभिर्देवि
शुद्धात्मा विजितेन्द्रियः ।
शूद्रोऽपि द्विजवत् सेव्य
इति ब्रह्माब्रवीत् स्वयम् ॥४७॥

But conduct entitles one to be honoured regardless of the birth. Thus, the necessity of birth and the possibility of honors regardless of birth are both brought out in rudra’s discourse.

A great Śiṣtottama & Ācārya that he was; he was able to engender only positive experiences in those who came to him; whoever they were or wherever they came from, provided that they came with honesty & good-will.