As panjandrum says, song lyrics are not necessarily meant to be understood, but at a guess, it seems to me it could be that the singer has done something bad in the past and so they are asking to be allowed to atone for their sins by reflection and comparison of themselves with some holy soul (God?) whom they are addressing.
My reasoning is:
redeem - The singer wants to atone/compensate for past wrongs by doing good
funereal - someone may have died, but more importantly I think the singer feels mournful and sombre.
mirror - gives an impression of reflection (thought) or comparison. Just as you would see your appearance if you look in a mirror, the singer is going to look upon the holy soul and realise how his/her own faults by comparison.
sactorum = of holy places. There might be a holy place where this reflection etc takes place.