from what i read online after you beat moms heart a beam of light is suppose to show up and take you to the Cathedral, but i just beat moms heart for the 2nd time and nothing showed up just the chest that ended the game for me
You have to beat Mom's Heart 10 times to have a guaranteed run at Sheol/Cathedral. Until then, access to those areas works like a Devil Room spawn: if you don't get hit on Womb 2 (and to a lesser extent, no-hit on the boss) then you have a higher chance of the room spawning. If a God Room spawns you can go to Cathedral, if the Devil Room spawns you can go to Sheol.
Just keep playing and you'll get the ability to play Cathedral/Sheol as much as you want.
If you grab a Joker Card (Tarot/Pills slot), keep it until you reach The Heart and use it sometime during that floor. It will warp you directly to the Devil/God room and allow you to bypass the boss completely and head on down to Sheol/Cathedral. I'm not entirely sure if the Cathedral is an option at this point for you as it only ever gave me Sheol until I killed the heart 10 times.
Also, the beam of light seemed to happen at random - sometimes I would just get Sheol, sometimes both, sometimes neither.