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The lyrics for Adoro te devote were composed by St. Thomas Aquinas. CPDL says the following about this hymn:
Eucharistic hymn (Meter 11 11. 11 11) attributed to Thomas Aquinas, with notable English versions by J. R. Woodford (whose version is in 10 10. 10 10 meter), John Mason Neale and Gerard Manley Hopkins. The traditional tune appears in the 1697 Processionale Parisiense pp. 192-3.
See Torrell, O.P., Saint Thomas Aquinas (vol. 1): The Person and His Work, ch. VII, § "The Office of Corpus Christi", §§ "The Adoro te" for details about its authenticity.