There are many questions that lead to the 1914 question:
Why did nobody come to the JW interpretation of the Bible in 1830? ... "Because it was not the time of the end, only after 1914..."
Why it was right for CTR to leave SDAs instead of 'waiting for Jehovah'? .... "It was different then, it was not time of the end which started in 1914"
JW's teach there is some special period after the 1914. During this period there are different "rules of engagement":
- before 1914: study Bible individually, question auhorities, create separate Bible study group, individual relationship to God matters
- after 1914: join the right organization, otherwise you are lost!
Maybe you can start from this angle and later they will discover how much they rely on 1914