A common question raised by skeptics is that everything in the world has a cause, so if God is the cause of the world, then who or what is the cause of God? Here we come to the specific definition of God- the One Who is the supreme cause of all causes, while He Himself has no cause. As He declares:
aham sarvasya prabhavo mattah sarvam pravartate
iti matva bhajante mam budha bhava-samanvitah
(Bhagavad-gita, 10.8)
“I am the source of all spiritual and material worlds. Everything emanates from Me. The wise who perfectly know this engage in My devotional service and worship Me with all their hearts.”
Here the word ‘sarvam’(everything) is significant, for that includes whatever might exist in the material and spiritual worlds, that means, anything or anyone other than Himself. Thus, nothing can be the cause of God, for there is nothing that exists which was not manifested by Him, and it would be illogical to consider a created being to be the cause of its cause. Moreover, the imposition that God must have a cause would lead to regressus ad infinitum, what would contradict the very definition of God too. This is described in sankhya in these words:
mule mulabhavad amulam mulam
(Sankhya-sutra, 1.67)
"This is so because the root cause of everything is not caused by another root cause."
The sruti confirms:
na tasya kascit patir asti loke na cesita naiva ca tasya lingam
sa karanam karanadhipadhipo na casya kascij janita na cadhipah
(Svetasvatara Upanisad, 6.9; Govinda-bhasya, 1.1.1)
“No one is His master. No one controls Him. He does not have a material body. He is the cause of all causes. He is the master of all masters of the senses. No one is His father. No one is His king.”
The smrti says:
isvarah paramah krsnah sac-cid-ananda-vigrahah
anadir adir govindah sarva-karana-karanam
(Brahma-samhita, 5.1)
“Krsna, Who is known as Govinda, is the Supreme Godhead. He has an eternal blissful spiritual body. He is the origin of all. He has no other origin and He is the prime cause of all causes.”
The conclusion is that everything other than God has an origin, thus by the indirect process if we find someone who has no cause, He is God.