I've defeated it lives about five times now (yes, the boss replacing mom's heart after its ninth defeat. Not mom's heart) and for some reason or another I never have access to Sheol. I only ever get the shining light leading me to the Cathedral which I've completed a number of times now.
Anyone know why this is? According to all my steam friends once I beat It Lives I should have guaranteed access to Sheol - but I don't.