That is a big question and I can’t go into all the details here. However let me point out a couple of fairly big issues that distinguish the two.
Christianity is all about a person - Jesus Christ - who lived and died and rose again. He made many claims about himself and backed them up by his actions. His ultimate claim is that he is God made human and that by dying on the cross he has opened the way for all of us to be reunited with God in perfect relationship. He did this because he loves us and as a result we need to do nothing to be made right with God - simply receive this gift.
The thing to note is that these are central issues of the Christian faith and if you do not believe them then you cannot be called a Christian. If any one of these is wrong then all of Christianity is wrong and is not worth following. This is what Paul (one of the writers in the bible) says:
That if you confess with your mouth, “Jesus is Lord,” and believe in your heart that God raised him from the dead, you will be saved.
If there is no resurrection of the dead, then not even Christ has been raised. And if Christ has not been raised, our preaching is useless and so is your faith. More than that, we are then found to be false witnesses about God, for we have testified about God that he raised Christ from the dead. But he did not raise him if in fact the dead are not raised. For if the dead are not raised, then Christ has not been raised either. And if Christ has not been raised, your faith is futile; you are still in your sins. Then those also who have fallen asleep in Christ are lost. If only for this life we have hope in Christ, we are to be pitied more than all men.
1 Corinthians 15:13-19
While the Jehovah Witnesses believe that Jeus lived and died, they do not believe everything that he claims about himself. They do not believe that he is God but has been created by God. He is still very important to God but not in essence God. They believe that he is in fact the archangel Gabriel become man.
And while they believed he dies and paid a rasom for the sins of humanity, they do not believe that he rose again bodily but spiritually and materialised a new body in order to appear before his disciples.
Both these claims are counter to the central claims of Christianity and, if in fact they are true, condemn Christianity as false and nt worth considering.
Jehovah Witnesses differ with Christianity in many other areas as well - some are important others are not so. However in these central issues, the differences mean that we cannot call them Christian.
So what do we say to them?
This is a very difficult question since there are no “tried and tested” approaches that will bring about change - especiaaly in a 15 doorstep conversation. However like with other people you want to tell about Christianity there are several important things you can do:
Pray for them and tell them that you are praying for them. After all conversion is God’s job not ours.
Love them. They are as much loved by God as you are and for us to treat them with respect and not slam the door in their face is important. Most witnesses are taught to expect opposition and door slamming will only reinforce this.
Try to understand them. Don’t let them reel off their well known and rehearsed speech but ask them to tell you about themselves - about how they came to be involved in the JWs, why they made the change and how it has been positive to them. Ask them about their personal relationship with Jehovah and so on. Make it personal, one to one. And then tell them about yourself and about how you became a Christian and about what being in relationship with God means.
With a Jehovah Witness in particular there are some helpful don’ts as well.
DONT get into a theological argument and trade verse for verse. They are rote taught theology by the Watchtower organisation and do not independently think about it.
DONT use their version of the bible. It is not a faithful translation of the original texts.
Hope this helps.
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Answers are kindly provided by our friends at Christianity.net.au