What if I stop believing in Christ?

Our listener asks, “I believe in once saved, always saved. How do you answer a person who says you can lose your salvation if you stop believing? They do not accept John 6:37 39. They say if you can be saved by believing and trusting Christ, then you can lose it the same way; by not believing and trusting in Him. God does not force us to continue in believing, therefore, we can lose our salvation by not continuing to believe and trust in Him. Thank you in advance.”