Are you worried because you find it so hard to believe? Don’t be surprised at the difficulty of faith, if there is some part of your life where you are consciously resisting or disobeying the commandment of Jesus. Is there some part of your life which you are refusing to surrender at his behest, some sinful passion, maybe, or some animosity, some hope, perhaps your ambition or your reason? If so, you must not be surprised that you have not received the Holy Spirit, that prayer is difficult, or that your request for faith remains unanswered...The person who disobeys cannot believe. Only if you obey can you believe.
Source: The Cost of Discipleship