At the age of 40, and as probably the most learned man of his era, Tsong Khapa joined the Kadam monastery of Radreng (rva.sgreng). Here, in 1402, Tsong Khapa completed his magnum opus, The Great Graduated Path (lam.rim.chen.mo), which was principally based on Atiśa's Bodhipathapradīpa, and has become the root text of the Geluk school. As elsewhere in his voluminous writings, Tsong Khapa emphasizes Prāsaṅgīka-madhyāmaka as the highest form of reasoning and stresses the correct understanding of relative reality as that which, while not possessing even a conventional own-being, can nevertheless be demonstrated by reasoning to be not non-existent. At the heart of The Great Graduated Path is the thesis that, while tantra may be necessary in order to become a fully enlightened Buddha, a prior study of sūtra is absolutely necessary for a preliminary development of wisdom and compassion. In another important work, The Great Graduated Path of Mantra (sngags.rim.chen.mo), which discusses the four classes of tantra, Tsong Khapa defines the relationship of tantra to sūtra as that between method and wisdom.
In 1408, Tsong Khapa established the Great Prayer (smon.lam.chen.mo), a New Year festival held in the Jokhang, which won him much devotional support. In 1409, Tsong Khapa had enough followers to found his own monastery of Riwo Ganden (‘Joyous Mountain’), and although initially calling his order the ‘New Kadam’, they soon became known as the Geluk. Tsong Khapa's views were similar to those of Atiśa, and it is unclear whether Tsong Khapa had reformed a Kadam tradition which had become lax, or whether the Geluk simply grew out of the Kadam under the impetus of his own personal renown. The founding of Drepung (ʾbras.sprungs) followed in 1416, and of Sera in 1419, the year of Tsong Khapa's death when his body was embalmed and placed inside a chörten at Ganden.
"Tsong Khapa." The Concise Oxford Dictionary of World Religions. . Encyclopedia.com. (August 21, 2018). http://www.encyclopedia.com/religion/dictionaries-thesauruses-pictures-and-press-releases/tsong-khapa
"Tsong Khapa." The Concise Oxford Dictionary of World Religions. . Retrieved August 21, 2018 from Encyclopedia.com: http://www.encyclopedia.com/religion/dictionaries-thesauruses-pictures-and-press-releases/tsong-khapa