During the time he served in the legislative halls, he did not neglect the higher honours of the vineyard of his Lord and Master. He was often at the family altar, in the closet and in the pulpit; and was esteemed as one of the most able, eloquent, and profound preachers of that eventful period. He was one of the brightest ornaments of the religion of Christ, and one of the strongest advocates of the cause of liberty. As a speaker, he was listened to with deep interest; as a logical and systematic debater he had few equals. His arguments were aposteriori, apriori and afortiori; leading the mind from effect to cause, from cause to effect, and deducing the stronger reasons. His memory was remarkably retentive, his judgment acute, and his perceptions clear. He was a member of the secret committee of Congress, the duties of which were arduous and delicate. He was a member of the committee appointed to co-operate with general Washington in replenishing and regulating the army; of the committee of finance, and of various other and important committees. Several eloquent appeals to the people from Congress recommending special days to be set apart for public fasting and prayer, were from his nervous and vigorous pen. The melting and burning manifesto, protesting against the inhuman and barbarous treatment of the American prisoners confined on board the filthy prison ships at New York, was supposed to have been written by him. From his mode of reflecting and reasoning, Dr. Witherspoon was prophetic in pointing out the results of propositions laid before Congress, and opposed all those that he believed would terminate unfavourably. Against the emission of continental paper money he strongly remonstrated. His predictions of its depreciation were soon verified. In March, 1778, one dollar and three quarters of paper money were worth but one silver dollar; one year from that time the rate was two for one; in five months after it was eighteen for one; the next year it was forty for one; shortly after, seventy-five for one; and in a few more months, one hundred and fifty for one; and finally became worthless.
Most of the measures he proposed when he commenced his career in Congress were either then or subsequently adopted with success, and those that he opposed unsuccessfully, terminated unfortunately in almost every instance. So closely and deeply did he investigate and probe every subject that came before him, that his powers of penetration became proverbial.