Once again il start by saying SN is the best show on telly and internet lol,anyway Dean never believed in God till he met castiel, but in earlier episodes like phantom traveller in season1 the demons reacted to Gods name,the bros said Christo(christ in latin) and the demon flinched,furthermore they have used holy water for as long as i can remember,they use crosses and catholic exorscism rituals, so after doing all this, dint the bros think for a second that the word christo,holy water,crosses,e,t,c come from heaven???or they might be another side,besides if there is a hell there has to be a chance of heaven.
I also wanted to address the people who find the notion of 4 angels seeing God hard to swallow,there are classes of angels and the archangels are among the lowest classes of angels and furthermore angels like castiel come below these.The 4 angels who have seen Gods face are the four highermost angels in heaven,they shield Gods true form,no one else ventures there.
On the case of mary winchester,if she was a hunter and she made a deal with azazel years ago,then she knew her time would be up really soon,so why in hell was she so surprised when old yellow eyes showed up 4 sam in the beginning,it was as if she had never seen anything supernatural.
Apart from that take care all you SN fans,the latest spoiler says they will be another sibling or sin for dean and sam,now i hope they wrong coz thats some straight bullshit.