In reply to:
he sees how vain going to the church pn Sundays is, it's just a habit, not real belief.
This is precisely my interpretation. I've speculated that the album represents a journey of the character from blind faith to more analytical, interpretive, individual faith, and Sunday is the perfect starting point. The religious are told to go to church on Sunday, so at the beginning of the album, the character does. He soon begins to realise, however, that he's not doing it for any rational reason, he's just doing it because he's told. He's doing it blindly. I'll even go a step further and speculate that this blind faith is indeed empty. It's going through the motions of religion and spirituality with little thought to what those words mean.
I quit going to church at the age of 8 because I recognised that for many in my church, the Sunday ritual was an empty, meaningless gesture. After being treated horribly and stabbed in the back by these supposed Christians, I realised that going to church was neither required for nor indicative of those with true spirituality. It was then that I chose to pursue my own spirituality on my own terms. I believe that Bowie's character will make a similar journey.