Everybody believes in something. Actually, in a lot of things. We may not know what those things are, but it shows. Your beliefs leak out in behavior. In religion, we would say that your faith is shown by the shape of your faithfulness.
We can’t see faith. While faith is confidence is something that cannot be seen, faith also is invisible to others. You can tell me about your faith, but I can’t tell if it is really your life doctrine, or merely a pretended dream. But what I can see is your faithfulness. And often, our faithfulnesses come across as faith(less)nesses, denying what we say we believe.
For instance, when I say that I believe in God who is Creator and Sustainer of the universe, and that He is intimately involved in the affairs of my life, even to the point of allowing/designing trials that are intended to strengthen my faith - and then, I’m hit with something unpleasant, and I moan and complain and get mad - well, my faith(less)ness has denied whatever I might have professed about my faith. I really believe something else, such as, if there is a god at all, he certainly owes me a pleasant and easy life; that he exists to serve me.
So faithfulness, properly expressed, can be a beautiful thing. But think also about this: faithfulness is not static. Think of a young child, learning to form words and explore. Yes, she wears a diaper which needs to be changed often, but she is being faithful to her nature, given her age and gifts. And we are delighted. But if that child reaches the age of 10 and still wears a diaper, we are dismayed. She no longer is faithful to what should be. So it is with Christians. We come to faith, and there is an infant-faithfulness that arises, including things like worship and fellowship. But along with the expectation of spiritual maturity comes the need for a more mature faithfulness, including the practices of service and sacrifice. Your faithfulness must grow.
Your faithfulness and mine will look different, even given similar beliefs. Your life situation, ordered by God, is different from mine; also your gifts and responsibilities. For some, it is gaining victory over public sins and picking up the fight against those not so obvious. It may be new service that pushes you to love deeply and give freely. It may be a trial that stretches the limits of endurance. But in all these situations, and more, your faith, if it is real, will show itself in faithfulness. And a lack of faithfulness is a denial of faith. Because, “faith without works is dead.”