Ask them why they teach that only 144,000 are going to heaven and are they all Jehovah's Witnesses. They'll read Revelation to you to 'prove' their point. Then say "...if only 144,000 are going to heaven, they must have been chosen a long time ago.." They'll tell you the sealing was generally closed in 1935. Ask them to prove that to you from the bible.
And if they try reading that scripture where Jesus said "...I have other sheep which are not of this fold, these I must also bring", point out the surrounding verses. He's talking about bringing the Gentiles into the fold, because didn't Jesus say that his mission was to preach to the House of Israel?
Watch their Christ-like attitude disappear before your very eyes.