Cassian believes in two things
Cassian Andor used to believe in two things: blasters and the rebellion.
He had been travelling around worlds since he was a kid and he had seen many things, planets covered in frost and ice, planets where the sun shone on an eternal summer, planets made of cities and metals and buildings, planets hinabited only by flowers and forests and plants. He had seen many things, mostly beautiful, mostly horrific. The horrific ones where always related to the Empire. He fights because he belives in the cause, he carries a blaster because he can trust no one but himself and the hands he was birth with. Which sounds silly, but in the end it doesn't matter, what matters is what keeps him alive.
Cassian Andor still believes in two things: the rebellion and Jyn Erso.
He still trusts his blaster, but it's not the only thing he trusts; he still believes in the rebellion, but it's thanks to Jyn Erso if he now hopes for a future.