Reason and God denote the same person - namely, a person who is omnipotent, omniscient, and omnibenevolent. But that does not mean that the words mean the same thing such that one could use them interchangeably (although if it is understood that both denote the same person, then I suppose they could be).
Let's say I am the richest person in the bar, but the person I am drinking with does not know this. They just think I am Bartricks. They see that I have finished my drink and want another. They know that Bartricks wants another drink. Do they know that the richest person in the bar wants another drink? No. If you asked them 'does Bartricks want another drink?' they would say 'yes'. If you asked them 'does the richest person in the bar want another drink?' they'd say 'I don't know'.
Reason is the name for the source of all (normative) reasons. That's what it denotes. Most people don't realize it's a person. It is and this can be discovered easily enough. But there you go. Once one understands that Reason is a person, what we say about Reason makes sense (listen to Reason, follow Reason, the imperatives of Reason and so on).
God is the name of a person who is omnipotent, omniscient and omnibenevolent. Reason will have those qualities. Thus Reason is God.
To go back to my analogy with myself, imagine that the person who owns the goldmine is, by virtue of this, the richest person in the bar, for that's the most valuable asset in these parts. The person I am drinking with does not know that the person who owns the goldmine is, by virtue of this, the richest person in the bar (for she does not know it is the most valuable asset in these parts). But she does know I own the goldmine. Does she know that the richest person in the bar wants a drink? No, she knows that Bartricks - the man who owns the local goldmine - wants a drink.
Similarly then, many people know that Reason wants them to do this and believe that. And many people also know that God is a person who is omnipotent, omniscient and omnibenevolent. Do they know that it is God who wants them to do this or believe that? No, not necessarily - not unless they realize that Reason is a person and that the person of Reason would have the attributes constitutive of being God.
Likewise, many of those who know that Reason wants them to do this and believe that, also believe that God wants them to do this and believe that. Do they know that Reason and God are one and the same person? No. Just as my drinking partner may know that the richest person in the bar wants a drink - someone has told them - and know as well that I want a drink, yet not realize that myself and the richest person in the bar are one and the same person.