UNSUSPECTING= If you are unsuspecting, you have no idea that something is happening or going to happen (you don't suspect anything).

HALLELUJAH= /(h)æləlu:jə/ (Hebrew word from the Bible, also spelled Halleluyah, Halleluiah or Alleluia) A joyful word of praise to God, something like "glory be to God" or, in modern language, "God rules!", "He's the best!". The strange J that we pronounce /j/ (like an Y) here is an old spelling that has survived because it was used in the standard Bible for the English (King James Version), but you can also use the I (or even Y), which fits better with its pronunciation. Spellings without H are also possible because you can also pronounce this word without /h/.
We can also use this word to express a great joy for something good that happened, specially after a long wait (e.g. "halleluiah! you finally passed your driving test!")

LORD= God is often called "the Lord" because he has power over everybody the same as in all times a lord (or king) had power over all the people living in his lands. You can also say "the Lord God", though it is a bit redundant.

OMNIPOTENT= All-powerful, having unlimited power (from Latin omnia=all things + potens= powerful)

REIGNETH= (old fashioned or literary) /reɪnɪθ/ Reigns, rules. A king reigns (exercises power) over his people.
In Old English, the third person singular of the present tense added –ETH /ɪθ/. But in the north of England, due to Danish influence (they were conquered by the Vikings), they started to add –ES, and that is the form that finally became standard a few centuries ago as -S, so today we say "he walks" and not "he walketh", though the –ETH form is still used in poetry and the standard KJV* Bible translation.  (*King James Version).

OUR LORD= God

HIS CHRIST= Jesus.
The word "christ" /kraɪst/ is the Greek translation of the Hebrew word "messiah" /məsaɪə/, which means "the anointed one" (the one chosen by God to lead His people). In ancient times God promised to send a messiah to save His people. Christians believe that Jesus was the messiah promised by God, so they called Jesus "the Messiah". Since most of the New Testament was written in Greek, He was called "the Christ", and very soon people mixed both names and started to call Him "Jesus the Christ" or "Jesus Christ", or simply "Christ" used as a proper name.
Latin people used the spelling CH to transcribe the Greek sound /x/ (which sounds like a strong "h", as in the Scottish "loch" or the Spanish "Juan"). The Greek word Xristos was pronounced /xristos/ and spelled Christus in Latin. Of course, most people pronounced it with a Latin accent: /kristus/. In English, most of those ancient Greek words originally pronounced with /x/ and transcribed with CH still keep the CH spelling but have the Latin pronunciation /k/, for example in: Christ, mechanic, technician, chaos, anarchist, monarchy, anachronism, chromatic, archaeology, archetype, archaic, architect, psychology, melancholy, technology, schizophrenia, character, chemical, chorus, chronometer. All these words, and many more, are pronounced with /k/ in English.

SHALL= (old fashioned or emphatic) Will.

FOR EVER AND EVER= An emphatic form of "forever", always, for all eternity.
So the sentence "He shall reign for ever and ever" means "He will be king eternally".

KING OF KINGS= The most powerful king and lord of all the other kings; or the one and only king.
Religious expressions like "king of kings" and "lord of lords" are actually a direct translation of a Hebrew grammar structure used to express superlative, so "king of kings" really means "the most important king".

LORD OF LORDS= The most important ruler (see above)