Can I deal myself 5 damage with sacrosanct if I only have 1 life? Or does my life limit what I can inflict on myself?
You can deal yourself 5 damage in this case. However, if you are incapacitated your cards will leave the game and you will not get to deal damage to another target. So long as you have Aegis of Ressurrection (or Fixed point) in play, though, you can survive going into negative HP and deal the full 5 damage to another target. (or more if you managed to take additional radiant damage this turn)