The setting is the great Westminster Assembly:
During the debate on church discipline…Erastians placed the power of excommunication in the hands of the civil authority. One of their leaders, ‘the learned Selden’ held forth at great length that Matt 18:15-17, the passage under dispute, contained no warrant for ecclesiastical jurisdiction, but concerned the ordinary practice of the Jews in their common civil courts.
Even the most erudite and able of the divines present were in no hurry to encounter such a formidable opponent. Samuel Rutherford turned to Gillespie and said “Rise George, rise up man, and defend the right of the Lord Jesus Christ to govern by His own laws, the Church which He hath purchased with His blood.” With every appearance of reluctance, Gillespie rose, gave first a summary of the previous speech, striping it of all its cumbrous learning, and reducing it to simple langugage. Then steadily, point by point, he completely refuted it, proving that the passage in question could not be interpreted or explained away to mean a mere civil court, and that the Jews both possessed and exercised the right of spiritual censures. The effect of Gillespie’s speech was so great as not only to convince the Assembly, but also to astonish and confound Selden himself, to whom Gillespie was a veritable enfant terrible. The Erastian leader is reported to have exclaimed in bitter mortification: “That young man, by this single speech, has swept away the learning and the labour of ten years of my life.”
Stuart McMahon quoted it at the Closing Lecture. The version above comes from the book Light in the North: the story of the Scottish covenanters by J.D. Douglas, p41. And a bit from yer man’s version not mentioned above…after the speech, the Scots looked at Gillespie’s notebook so they would be able to preserve the great speech, but “they found nothing but the words Da lucem, Domine – Lord, give light – and similar brief petitions for guidance.”