The reason their belief doesn't make much difference to them is because it's not a true belief. It's only a vague, weak belief that hasn't penetrated their hearts and minds, and therefore it doesn't influence every part of their lives.
This doesn't mean they don't believe that God exists - because they probably do. If sickness comes to a loved one, they'll probably turn to God in prayer (although they may not pray otherwise). They may even defend their belief in God if someone who fiercely opposes religion attacks them. But they don't really know God, nor is He important to them. In other words, they know about God - but they don't really know Him.
The Bible, however, tells us not only that God exists, but that He loves us and wants to come into our lives and become the center of everything we do. And this is possible because God sent His only Son into the world to erase the barrier between us and God - the barrier of sin. Our greatest need is to turn to Christ in faith and trust, and commit our lives to Him.
Jesus once warned us not to be like the person who hears the word of God, "but the worries of this life, the deceitfulness of wealth and the desires for other things come in and choke the word" (Mark 4:19). Don't let anything crowd out God, but make Christ the foundation of your life - beginning today.