What is the authority for any concept in spirituality?

The Veda is the only authority which was spoken by the Lord and which was protected by recitation without any pollution. Vyasa decided this in his third Brahma Sutra “Shastra Yonitvaat”. Shastra means the Veda, which was spoken by the ruler (Shasaka). The Gita also says that Shastra is the authority. It is said that the Gita is the essence of the Vedas. The cane sugar juice is in the sugar cane. But sugarcane is not in the cane sugar juice. Similarly the meaning of any verse from the Gita must be found in the Veda. All the Veda may not be found in the Gita. Therefore if the meaning of a verse in the Gita is not traced to the Veda, Lord Krishna did not speak such a verse. Such a verse must have been inserted later on [by some scholar]. Therefore only the Veda can stand as the authority.

Any Sanskrit verse from any holy book (Smriti) should not oppose the Veda. Any point, which is not said in the Veda, cannot be accepted even if Smriti (Sruti Smriti Virodhetu…) presents it. Only that logic, which does not contradict the Veda, can be accepted as an authority according to Shankara (Sruti Matah Tarkah). Only that experience, which does not contradict the Veda, can be accepted because the experience of a foolish man cannot be accepted as an authority. Therefore Smriti, logic and experience, are accepted only when they follow the Veda. The Veda is the constitution. A lawyer has to refer to the constitution throughout his argument in the court. Only two lawyers are allowed to argue in the court. Similarly only two scholars of the Vedas and Shastras can argue on a concept. You can easily preach to a layman. You can also preach to a Vedic scholar. But even Lord Brahma (Brahmaapi Na Ranjayati) can never convince a fellow who has little knowledge. A layman should employ an advocate on his behalf in the court. Similarly a layman or a person with little knowledge should not argue with a Vedic scholar (Vivaadascha Samayoh).