I grew up as a Jehovah witnesses, my whole family are witness i live with my father and brother who have that state of mind, im about the only one in my dads side of the family who is classified as christian. My grandparents are elders in the church, it is very hard to speak to them about the cross ,and the trinity. I love Jehovah witnesses simply cause of the fact there very devoted, theyve just been misinformed. They way to talk to them is to point them back to Jesus always talk about Jesus, who he is, how it was possible for him to die for us, talk about us been sinners through adam. Stay away from, talking about the 144,000, dont use the word"cross" use the original greek word for cross "stauros" which means torture stake but we know he died on a cross, if they argue about the cross, give them this verse from their on bible the"New World Translation" John 20:25 Consequently the other disciples would say to him: "we have seen the Lord!" But he said to them:"Unless I see the in his hands the print of the nails and stick my finger in the print of the nails and stick my hand into his side, I will certainly not believe". Now notice the nails imprint comment, if he died on a stake like they show in many images, then thomas would had said the nail. If they try to justify that then point them to mathew 27:37 also they posted above his head the charge against his head, the charge against him, in writing"This is the king of The Jews" If you notice there images of christ being nailed to a stake, the sign post is above Jesus hands not head. One more passage i would equip you with, first of all ask God when to use this, cause you might want to talk about Jesus and how he fullfill the Law and how we are sinners first (or however god guides you), then when your about to close ask for there bible, and go to John 20:28 In answer Thomas said to him "My Lord and My God!" Hallelujah! God sent them to you. They might think that theyre there to convert you but theyre there to know Jesus though you, Show them Love and find a common denaminator Jesus.