Major Mahayana sutras on universal Buddhahood, skillful means, and Buddha-nature.
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Lotus Sutra — The Saddharma-puṇḍarīka — The Lotus of the True Law — the foundational Mahāyāna scripture of universal salvation, the teaching that all beings will attain Buddhahood, in H. Kern's 1884 translation.
Tathagatagarbha Sutra — The Buddha reveals, through nine similes, that all sentient beings possess the tathāgatagarbha — the embryo of awakening — hidden within them like gold in filth or a king in a lowly womb. Translated from the Chinese of Buddhabhadra (佛陀跋陀羅, 420 CE). Taishō Tripiṭaka Vol. 16, No. 666.