GOTTA= (coll.) have got to.
- I gotta go = I've got to go / I have to go / I must go.

SOMEWHERE= In some unspecified place.

WHY NOT...?= We use this to make a suggestion.
- Why don't we play chess? = Let's play chess.
- We need a car. Why not your car? = I suggest we use your car.

SOMETIME= In some unspecified moment, some day.
(other compounds: somebody, something, somewhere, somehow)

CATHEDRALS= (in Christianity) The main temple in a diocese where the bishop is. They are usually the best richest and most beautiful temple in the area, serving a city and all its area of influence. (see picture)

THIRD WORLD= Underdevelopped countries, poor countries.

MISSIONS= A mission is a place without a steady priest where a Christian person or group helps people in the area know about God and also helps them in any other material way they need, including helping them out of poverty. The temple there (if there is any) is usually a poor construction made with basic materials from the area, often no more than a wooden chapel.
By contrasting the rich impressive cathedrals with the poor shabby missions, the singer wants to include all varieties of places for cult.

CRASHING WAVE= An ocean wave is a crest of water moving along the surface. If they are big enough, when they reach the coast they crash powerfully and break against the rocks or sand: that is a crashing wave. Here it is used metaphorically to refer to a huge movement of people who want to change the world and crash into any problem, "washing" them away.

TONIGHT'S THE NIGHT= This is the night we're waiting for.
(we can also say "today's the day"). He talks about "tonight" and not "today" because he's contrasting our dark present times (night) with the glorious future that is waiting for us (day).

SINNERS= A sinner is a person who has sinned. To sin is to do something morally bad which is damaging yourself or/and others or/and is an offence against God.

SAINTS= A saint is a good person who is in heaven, but we can also use "saint" for living people who are extremely good.
When we use the construction THE + ADJective we refer to the group of all people having that quality, so "the sinners" are all the bad people and "the saints" are all the good people. We are all sinners because nobody's perfect, but when we contrast "the sinners" and "the saints", we're usually talking about good people and bad people. Here, the singer wants to include everybody, all humanity, people of all kinds, good and bad.

COLLIDE= Clash, crash together with a violent impact, produce a collision.

DISPLAY= A show, a public exhibition, an open view that everybody can see.

STEP ACROSS THE LINE= To cross the line (walking), to go to the other side. "The line" is a division, the division separating the two worlds this song is talking about, which, in other words, is: friends and "enemies", but the song really wants to say that there are no enemies, we're all a big family.

A CITY WITH ONE KING= The singer is talking about how all the people in the world can unite to make a better world, a better society. That perfect world with no wars, no pain, no suffering and no injustice is described in the Bible and called The New Jerusalem. Some people think the New Jerusalem refers to heaven, so we'll all go to that place when we die (if you're good, that is), and other people think it refers to a perfect society on earth, a "heaven on earth" that people in the future will be able to enjoy, but we have to start working on it now if we want that place to ever come true. The singer here is talking about this second city, the perfect society that we must build all together. In any case, the Bible says that city will have only one king: God, and it is often referred to as "the Kingdom of God".

ON OUR KNEES= Kneeling down (see picture). To be kneeling down is a common posture of worship (adoration) in many cultures. If the city he's talking about is a sacred city, with God as a "king" ruling there, people will metaphorically be "on their knees" worshipping God, because that will be a holy place.

CHOICE= Election, the act of choosing.

WE ARE ONE CHOICE FROM TOGETHER= The only thing that separates us (all humans) is one choice. If we choose to be united then we will all be one nation and work together.

AS FAMILY= /fæməlɪ/ like a family, as if we all were one big family.

GLORIOUS= Wonderful, amazing, fantastic (in fact "glorious" suggests something even more fantastic that those synonyms). But the word "glorious" comes from "glory", a word often used to describe the splendour or praise referring to God, so "a glorious display" makes us think in something sacred, holy.