"To try to keep it short you have to get them to acknowledge that the only way to know what a religion teaches is to read its authoritative documents. Then you can take them right to verses in the Koran that command Jihad - there are hundreds.
The answer to the question of how come so many Muslims don't engage in Jihad is simple. Jihad - and many other Islamic moral rules - conflict with God's laws which Paul tells us are written on the heart of all men (When the Gentiles do by nature...). To engage in Jihad, one must first become convinced of the teaching of Koran and Hadith that the only sure way for a Muslim to go to heaven is to die fighting for Allah.
A reader in Britain sent me a link to a site that is really helpful for showing people what Islam truly teaches. It is called Islam Q & A (for question and answer) and is done for Muslims by Muslims. Take a look at what it says about jihad at this page: http://63.175.194.25/index.php?ln=eng&ds=qa&lv=browse&CR=309&dgn=4.
If that link doesn't work you can get there by going to www.islamqa.com. Click on English. Click on Islamic Politics in the lower right. Click on "Rulings on Jihaad (sic)" on the top right of the next page."
Sorry for the double tap. My mouse has a hair trigger and I drink a lot of coffee.