Rig Veda 8.6.10 says
I from my Father have received deep knowledge of the Holy Law I was born like unto the Sun.
If we ponder over deeply, we can understand that it is talking about spiritual re-birth after getting over ignorance.
It indicates twice born or dwija.
Rig Veda I.60.1, dedicated to Agni, mentions dwija - dvijanmānaṃ
vahniṃ yaśasaṃ vidathasya ketuṃ suprāvyaṃ dūtaṃ sadyoartham | dvijanmānaṃ rayimiva praśastaṃ rātiṃ bharad bhṛghavemātariśvā ||
As ’twere Some goodly treasure Mātariśvan brought, as a gift, the glorious Priest to Bhṛgu, Banner of sacrifice, the good Protector, child of two births, the swiftly moving envoy.
My question is when one becomes dwija, after getting enlightenment/realisation, why are brAhmana, kshatriya and Vysya called dwija, after performing upanayana ritual?