When you truly know something, there is no need for belief. You know that your heart is beating; there is no need to believe that your heart is beating. You know when you are breathing. There is no need to believe that you are breathing; you are aware it is happening. When you know God (whatever your definition of God entails) is within, there is no need to believe in God. You know it!
When your mind is still and your inner dialogue has died down and you can separate from your thoughts; in that stillness you will know. There is no need to convince others of what you know. Know what you know and just be. With belief, there is always room for doubt to enter. When you know something, really know it, there is no doubt.