Definitions:
Classics with a capital "C" refers only to works written in Classical Greek and Classical Latin, the languages of Demosthenes of Athens Greece, and Cicero of Rome. Thus the New Testament is not a Classic because it is written in Coine or "gutter Greek". or "ghetto Greek". Cicero would choke or laugh if he read it. The Old Testament is not a Classic either because it is not written in either Classical Greek or Classical Latin. Thus the Christian Bible, the Jewish Torah, and other religious books are not classified as Classics. Isaac Newton's book on Gravity is a Classic because it is written in flawless Ciceronian Latin, even though he wrote it in 17 century England. Many of the great scientific works of the Renaissance are also Classics for the same reason. Homer, Virgil, Epicurus, Epictetus, Plato, Aristotle, etc, are all considered Classics.
classics with a lower case "c" refers to all other ENDURING works of literature in any language. Enduring means that they have stood the test of time and continue to be read from generation to generation around the world: without support from proselytism (religious texts handed out through conversion processes). Shakespeare is classified as a classic, but not Classic. Spenser, Darwin, Einstein, Malthus, Gibbon, Hume, Adam Smith, etc, are all considered classics. The great religious texts of the civilizations of antiquity are considered classics, but not Classics. Thus the Torah, Koran, Old Testament, New Testament, Zend-Avesta, Bhagavad Gita, Nihon Shoki and Kojiki are classics, not Classics.