"We're not invited to believe doctrines, but only to believe in Jesus."
You may or may not agree with this assessment, but within the evangelical church at large (that segment of Protestanism that claims to believe the Bible is the inspired Word of God) there is a growing theological liberalism. It's not as advanced as the theological liberalism that has rung the deathknoll of mainline denominational churches, but it can be subtly detected in statements like the one above. Sadly, sometimes we must gently engage in apologetics with professing believers.
When another Christian says this, first we must not judge his or her motives. Assume he or she has good intentions behind such a statement and acknowledge that, simply showing this person gentleness and respect. You can say something along the lines of: "I think I see what you're getting at -- that churches often divide and split over disagreements about the teachings of the Bible. It's always sad when that happens, but here's why I think we can't separate doctrine from belief in Jesus."
The word doctrine simply means teaching. We are invited to believe in Jesus and as the Apostle Paul told the Christians at Galatia, the gospel (the message of the life, death and resurrection of Jesus) wasn't taught to him by any man, but by Jesus Christ himself. In other words, we are invited to believe in a specific Jesus, the One found in the pages of Scripture, the One revealed by God Himself.
Lots of people say all kinds of things about Jesus, which is why doctrine cannot be separated from receiving Jesus. The gospel writers teach us (give us doctrine) about a certain Jesus. The Jesus presented in the pages of the Bible is fully human and fully God, worked miracles, taught with the authority of God, said He came to seek and save the lost, willingly gave up His life to be crucified on a cross and then rose from the dead three days later because He taught of Himself that He is the resurrection and the life and that whoever lives and believes in Him will never die. Then Jesus Himself asked, "Do you believe this?" In other words, do you believe these specific things about Me?
Even Jesus was concerned about what people believed about him. This means Jesus was concerned about doctrine. "Who do you say that I am?" He's concerned about what people think of Him because it's vital to know who He really is and what He really accomplished through His work.
Jehovah's Witnesses believe in Jesus. But should we leave it at that and move on? Not if we care about Truth. Doctrine is concerned about the right teaching about Jesus and His gospel. Was He fully human and does it matter? Was He fully God and does it matter? Did He physically rise from the dead or just spiritually rise from the dead? Does any of that matter? Can't we just believe in Jesus?
Well, I hope you see it does matter because there are competing gospels about Jesus. That's why the Apostle Paul was so adament about anyone who preached (taught) a Jesus other than the One he taught to the Galatians.
Doctrine matters so much that Paul said that anyone who teaches a different Jesus other than the One the Apostle's taught is going to be condemned by God Himself. That's taking teaching (doctrine) pretty seriously.
To separate believing doctrines from believing in Jesus is a false distinction. They simply cannot be separated if you want to know the true Christ, the Living One, risen from the dead.