Jamieson, Fausset and Brown on Jeremiah 29:15

15. Because--referring not to the preceding words, but to Jr 29:10, 11, "Jehovah saith this to you" (that is, the prophecy of the continuance of the captivity seventy years), "because ye have said, The Lord hath raised us up prophets in Babylon," namely, foretelling our speedy deliverance (this their prophecy is supposed, not expressed; accordingly, Jr 29:16-19 contradict this false hope again, Jr 29:8, 9, 21). He, in this fifteenth verse, turns his address from the godly (Jr 29:12-14) to the ungodly listeners, to false prophets.