"Sastra means the transcendental literature, not ordinary writings. That is sastra. Just like Bhagavad-gita is sastra, Veda is sastra, or Bible, sastra. Scripture means given by God or His authorized representative. That is sastra."
Srila Prabhupada Room Conversation, 07-18-73, London
"Srila Narottama dasa Thakura says, sadhu-sastra-guru-vakya, cittete kariya aikya. One should accept a thing as genuine by studying the words of saintly people, the spiritual master and sastra. The actual center is sastra, the revealed scripture. If a spiritual master does not speak according to revealed scripture, he is not to be accepted. Similarly, if a saintly person does not speak according to the sastra, he is not a saintly person. Sastra is the center for all. Unfortunately, at the present moment, people do not refer to the sastras; therefore they accept rascals as incarnations, and consequently they have made incarnations into a very cheap thing. Intelligent people who follow Sri Caitanya Mahaprabhu's instructions and the instructions of the acarya, the bona fide spiritual master, will not accept a pretender as an incarnation of God. In Kali-yuga, the only incarnation is Sri Caitanya Mahaprabhu. Imitation incarnations take advantage of Sri Caitanya Mahaprabhu. The Lord appeared within the past five hundred years, played as the son of a brahmana from Nadia and introduced the sankirtana movement."
Caitanya-caritamrta, Madyam lila 20:352
"So if this confidential knowledge, one who preaches without any compromise, he is the confidential servant of Krsna. There is no compromise. This is real religion. ...Krsna says, na ca tasmad manusyesu kascid me priya-krttamah. So this is the person who has received the authority to draw mercy water from the ocean of mercy of Krsna. Saksad-dharitvena samasta-sastraih. And what Krsna said five thousand years ago, the same thing Caitanya Mahaprabhu said, same thing. There was no change, as there was no change between the statement of Visvanatha Cakravarti Thakura and Narottama dasa Thakura. Sadhu sastra. As sastra, there is no change. Not that "Modernize. The sastra should be changed." No. That is nonsense. That is not sastra. Sastra cannot be changed. "Circumstantially, it will be changed, seasonal changes." No. That is not sastra. Sastra means it is perpetual. What Krsna said five thousand years ago or Krsna said some forty millions of years ago to the sun god... Imam vivasvate yogam proktavan aham avyayam. He says, "I am talking to you that puratanam yogam." Not that "Because it has passed millions of years and now it is a different time, so I will have to change." No. He said, "I am talking to you that very old system." ...And the sadhu... Sadhu means who follows the sastras. He is sadhu. He also does not change. Sadhu, sastra. And guru? Guru means who follows the sastra and sadhu. So there are three, the same."
Srila Prabhupada Lecture on Srimad-Bhagavatam, 11-30-76, Vrindaban
"....Everyone has got some scripture. The Hindus, they have got Vedic scripture. The Muslims, they have got Koran. The Christians, they have got Bible, Old Testament, New Testament. Similarly, there are many other religious sects, they have got their own scripture. So Baladeva Vidyabhusana says that vedoktasya dharmasya. Dharma means the rules and regulations as they are prescribed in the scriptures.
Just like state laws. State laws, there are some rules and regulation in the lawbook, in the statute book of the particular state. Similarly, dharma, another meaning of dharma is, it is the law of God. Maybe differently described in different countries according to different climatic condition or situation. But in every religious scripture the obedience to God is instructed. That is a fact. No scripture says that there is no God and you are independent. "
Srila Prabhupada Lecture on Bhagavad-gita, 06-13-68, Montreal
Bhaktivedanta Book Trust. HDG A.C. Bhaktivedanta Swami Srila Prabhupada.