Eternal life is truly given, but it can be freely rejected because grace doesn’t cancel free will. Salvation is a real relationship, and relationships can be abandoned and they can improve. At the core, I read the words 'he who hears My word and believes' as an active, living faith. I also read numerous warnings in scripture for believers about falling away. The promise applies to the one who remains in Christ and not to a past moment of belief detached from perseverance. It's a theological difference. But I think you already know this. Catholics understand faith as a journey, a daily walk.