I know Raiden (from 'Metal Gear Revengeance') has a sword that can cut through anything but I wanted to know what the underlying justification was for this ability?
What technology allows his blade to cut through anything?
Raiden's weapon was a high-frequency blade.
A high-frequency blade was a sword reinforced by a powerful alternating current and resonated at extremely high frequencies. This oscillation weakened the molecular bonds of anything it cut, thereby increasing its cutting ability. Striking, cutting, and thrusting attacks were performed with the blade, with the sharpened edge and point used for lethal attacks, and the blunt edge used as a clubbing weapon. Proficient users were also able to wield the blade with such accuracy and speed that they were able to deflect bullets.
Basically, it uses sound waves and electricity to cut through things (the latter of which you can see in the picture).
I think you're mixing up two separate questions here: Raiden's High-Frequency blade operates as per Jonah's answer, but the Muramasa is Jetstream Sam's blade, not Raiden's:
This is the Muramasa, and as the final fight demonstrates, it's able to cut through things that Raiden's default HF Blade can't. The reasoning for this is its heritage: according to the wiki:
According to Blade Wolf the sword's high-frequency conversion carried over the already excellent properties of the original sword, making it extremely powerful.
So, if you were asking why the Muramasa was able to cut through anything instead of the standard HF Blade, it's because of
magic superior craftsmanship.