He doesn't believe in magic just as he doesn´t believe in fairytales, or, for that matter, fairytale endings. Wishes, on the other hand, may come true-- if one ensures all necessary conditions are met. Is it impossible? Then little use in wanting, let alone trying.
However, the term "impossible" has been narrowed considerably since he first heard the dulcet tones of a name that conjures up Lombardian villages rather than weapons of mass destruction.
Nonetheless, such devices are among Rambaldi´s creations, if mere means to an end. World domination? A terrible cliché with fictional villains, but a reasonable-- admittedly ambitious-- goal for the ones he´s with.
Finding the lever to lift the world off its hinges is simply a matter of time now. Minor inconveniences notwithstanding, everything has gone according to plan-- Sloane´s, Irina´s, ultimately, a medieval Italian´s.
Sark does, upon occasion, believe in God. He just doesn´t serve Him.