Historically the Russians have long had close ties with Mt Athos, including an invasion of it in the 1920s (?).
Whatever the thug in the Kremlin's motives are you can be certain they are aimed squarely at Advantage Russia or Advantage Putin at some point in the future.
What better place to undertake nefarious activities against the West than a strategically placed severely off-limits security mad religious community where every visitor is documented to the hilt?