16
Śrī Caitanya-caritāmṛta
[Antya-līlā, Ch. 16]
śūdre tu yad bhavel lakṣma 
  dvije tac ca na vidyate 
  na vai śūdro bhavec chūdro 
  brāhmaṇo na ca brāhmaṇaḥ
“If the characteristics of a  brāhmaṇa  are found in a  śūdra  and not in a  brāhmaṇa , that  śūdra  should not be known as a  śūdra , and that  brāhmaṇa  should not be known as a  brāhmaṇa .”
Similarly, in the  Vana-parva , Chapter 211, it is said:
śūdra-yonau hi jātasya 
  sad-guṇānupatiṣṭhataḥ 
  ārjave vartamānasya 
  brāhmaṇyam abhijāyate
“If a person born in a  śūdra  family has developed the qualities of a  brāhmaṇa , such as  satya  [truthfulness],  śama  [peacefulness],  dama  [self-control] and  ārjava  [simplicity], he attains the exalted position of a  brāhmaṇa .”
In the  Anuśāsana-parva , Chapter 163, it is said:
sthito brāhmaṇa-dharmeṇa 
  brāhmaṇyam upajīvati 
  kṣatriyo vātha vaiśyo vā 
  brahma-bhūyaḥ sa gacchati
ebhis tu karmabhir devi 
  śubhair ācaritais tathā 
  śūdro brāhmaṇatāṁ yāti 
  vaiśyaḥ kṣatriyatāṁ vrajet
na yonir nāpi saṁskāro 
  na śrutaṁ na ca santatiḥ 
  kāraṇāni dvijatvasya 
  vṛttam eva tu kāraṇam
“If one is factually situated in the occupation of a  brāhmaṇa , he must be considered a  brāhmaṇa , even if born of a  kṣatriya  or  vaiśya  family.
“O Devī, if even a  śūdra  is actually engaged in the occupation and pure behavior of a  brāhmaṇa , he becomes a  brāhmaṇa . Moreover, a  vaiśya  can become a  kṣatriya .
“Therefore, neither the source of one's birth, nor his reformation, nor his education is the criterion of a  brāhmaṇa . The  vṛtta , or occupation, is the real standard by which one is known as a  brāhmaṇa .”