Disbelief is sin. Sin is not just an external wrong act. Sin is anything that places God as less than He ought to be. This brings us to a very different understanding of the standard we need to attain to and of our need for repentance. Sin is not just doing bad acts.
Faith is a choice.
We tend to feel that our level of faith is dictated by what we feel. Faith is neither a passive state nor a wishy-washy hope for the future. It takes us past our senses and the natural realm we feel and understand. It reaches out to grasp hold of what we can only know exists by supernatural revelation. Note that the tenses are not both in the future.