Ask them to tell you what the "good news" is.
After you hear 45 minutes or so about bad world conditions, Jehovah removing all wickedness and bringing in his kingdom, a paradise earth etc., turn them to 1 Corinthians 15:1-4, read it and ask them why they talked for 45 minutes explaining to you what the "good news" is, but never mentioned the death of Jesus Christ for our sins and His resurrection from the dead, and salvation by faith in those things - which Paul writes is of "FIRST importance."
Now, brothers, I want to remind you of the gospel I preached to you, which you received and on which you have taken your stand. By this gospel you are saved, if you hold firmly to the word I preached to you. Otherwise, you have believed in vain. For what I received I passed on to you as of first importance: that Christ died for our sins according to the Scriptures, that he was buried, that he was raised on the third day according to the Scriptures,
Then, if you really want to drive the stake into their hearts, refer over to Galatians 1:8, 9:
But even if we or an angel from heaven should preach a gospel other than the one we preached to you, let him be eternally condemned! As we have already said, so now I say again: If anybody is preaching to you a gospel other than what you accepted, let him be eternally condemned!