In Which Knowing Alone isn't Enough

[crystal_clear]

(فَإِن تَوَلَّوْاْ فَقُلْ حَسْبِيَ اللّهُ لا إِلَـهَ إِلاَّ هُوَ عَلَيْهِ تَوَكَّلْتُ وَهُوَ رَبُّ الْعَرْشِ الْعَظِيمِ (9:129

9:129 (Asad) But if those [who are bent on denying the truth] turn away, say: "God is enough for me! There is no- deity save Him. In Him have I placed my trust, for He is the Sustainer, in awesome almightiness enthroned."

They say knowledge is power. Faith is perhaps even moreso.

So why is it so hard to believe in what one knows is the truth, the only way to salvation?