What Are Sins Against Faith? ...

 You should understand that we are all people of faith and that with different systems, it’s not a matter of faith vs. reason, but faith vs. faith. Faith is trusting in what you have good reason to believe. Doubt is not a sin against faith but a necessary component. No one has perfect faith: God requires sincere, unfeigned faith. Note: even Christians can be guilty of these sins. A sin against that would obviously be anything done in “bad faith” or demonstrating “no faith.”

1. For instance, having blind faith or not knowing why you believe or believing for no good reason at all. Not knowing what or why you believe is a way of not believing at all.  Faith without rational evidence is blind faith! 
2. Another would be the escapism and crutch of skepticism or of not accepting faith at all as a system to find truth, though it is a philosophical fact that all knowledge begins in faith and is contingent. You must always commence with some presupposition you cannot prove or disprove.
3. Nihilism is another anti-faith belief system whereas one denies truth can be known or anything has real meaning at all, or that even nothing makes sense at all or has purpose or even can be known; basically belief in nothing at all.
4. Postmodernism is a threat that denies absolute, transcendent, objective truth can be found or established at all; basically, to them, all truth is “relative.” The catchphrase is “That may be true for you, but not for me!”
5. We live in a post-faith era whereas Secular Humanists believe that faith is the enemy and that the only reliable tool for gaining knowledge is the scientific method. Therefore, one must observe it or be able to measure it for it to be true. But they don’t realize they are putting faith in the scientific method, their own power of reason, other scientists, materialism, and naturalism.
6. New Age or New Spirituality says we can find truth in ourselves and the “God within.” There is no universal truth at all and dogma is unofficial and personal.
7. Pragmatists are also anti-faith in that they say the measure of an idea or system is not whether it is true but its results and effects.