The first official Thanksgiving Proclamation was made November 1, 1777, by the Continental Congress. Samuel Adams wrote it. Here is part of it.
Forasmuch as it is the indispensable duty of all men to adore the superintending providence of Almighty God; to acknowledge with gratitude their obligation to Him for benefits received...together with penitent confession of their sins, whereby they had forfeited every favor; and their humble and earnest supplications that it may please God through the merits of Jesus Christ, mercifully to forgive and blot them out of remembrance...it is therefore recommended...to set apart Thursday the eighteenth day of December next, for solemn thanksgiving and praise, that with one heart and one voice the good people may express the grateful feeling of their hearts and consecrate themselves to the service of their Divine Benefactor...acknowledging with gratitude their obligations to Him for benefits received. ... To prosper the means of religion, for the promotion and enlargement of that kingdom which consisteth 'in righteousness, peace and joy in the Holy Ghost'.