Sunday Night Lyrics
if we hadn't taken shit seriously
then the present would be alright
patterns and habits continue but I've got soul
Can you imagine a world where there is no depth
absence of color and nothing to test our heads
I've been warning her all night
she's been crying her eyes out
imagine how different tomorrow would be
if we knew where to draw the line
I broke your promise and you flooded any chance
I don't intend to rewrite what belongs to you
Oh I was born just so you would learn how to choose
I've been warning her
she's been crying her eyes out
We were in the right place at the wrong time
if only you could read my mind
read my mind, read my mind, read my mind
No one I know up there will stand beside me.

Favorite song from them! I feel like it is a friendship that wants to be more than that, but the current situation they are in is making that not the best choice.
last 2 lines throws that kind of off though or I'm not making the connection with it.

Theirs choices brought out a different result in their relationship. He's thinking if they hadn't made a big deal out of their predicament then they would still be together.
Although together, the other attractive women he sees he flirts with. But no matter how far he goes with his flirting habits, he is still loyal to his main girl. His explanation is for her to imagine a world without flair and where nothing you do is wrong, and with nothing to test the outcome of their actions.
(Chorus) they've had a heated argument to which she was crying and what probably ended their relationship.
She left immediately knowing that he couldn't support himself on his own. He broke a promise to her disconnecting any chance to get back with her. He isn't planning on entering a relationship with another girl because he feels strongly that he belongs to her, and he's stating that he was born so that SHE can choose him over all the other guys in the world.
(Chorus) - - -
The ending verses imply's that they've changed, and he's a different person full of regret. Also, he was either at the same Church as her (On a "Sunday Night") or at her Wedding. And the people there who share the same views as him won't stand in his way.
As far as "No one I know up there will stand beside me" he's probably referring to the bride's maids and the groomsmen, and how they won't back up his intentions, or him being frustrated that God and the angels won't give him another chance.
My favorite song by them, Feels~