“Once you become convinced that compassion is the excellent key to the path through such proof as reason and limitless scriptures, why would you not hold the spirit of enlightenment together with its root – compassion – to be the supreme instruction? The teacher Shang-na-chung said, “Even though I asked for instructions from the Great Elder [Atisha], he said nothing except: ‘Renounce the world; cultivate the spirit of enlightenment.’” Upon hearing this, Geshe Drom-don-ba was embarrassed for him and said, “You received the Great Elder’s ultimate instruction!” Geshe Drom-don-ba understood the core of the teaching.”
From The Great Treatise on the Stages of the Path to Enlightenment by Je Tsong Khapa. As translated in: The Great Treatise on the Stages of the Path to Enlightenment: The Lam Rim Chen Mo by Tsong Khapa. Translated by the Lamrim Chenmo Translation Committee. Snow Lion/Shambhala Publications.