The theological symbolism (and LOTR parallels) of "Mission: Impossible - Dead Reckoning Part 1" [SPOILERS]

Just as in the case of Ilsa Faust (whose last name symbolized her "deal with the devil" with the corrupt MI6 leader Atlee) Grace's name is symbolic. She is introduced as a criminal, and Ethan Hunt repeatedly offers her a choice which he says every member of the team has been given at some point in the past (and which Benji and Luther later speak about). If she chooses to put her criminality behind her (repentance) and join the IMF (conversion), her criminal past (sins) will be forgiven (grace). At the end, she accepts the offer and fulfills her name.

It's also symbolic that the two halves of the key are cruciform (and referred to as such in one line of dialogue). One of the Russian sailors at the beginning prayerfully grasps half of the key (which is hanging around his neck) like a crucifix when the first (illusory) torpedo is about to hit their submarine. Also, Kittridge mentions "the cross you've been left to bear" in the closing monologue to Ethan.

The villain is named Gabriel because he serves at the messenger angel of The Entity, which is (as Ethan mentions in one line) his god.

The Entity also parallels the One Ring from "The Lord of the Rings". Almost everyone wants it in order to wield its power, but it has a mind of its own and is too powerful for anyone to be trusted with. One character (Gabriel) has become a slave to it, while one character (Ethan), aided by a fellowship of friends, seeks to destroy it, which requires a quest that involves bringing some metal to a particular location. This is similar to how how the mission in "Top Gun: Maverick" was inspired by the trench run in "Star Wars".