Why be good?

[Esquilax]

What I am asking is: what is the BASIS for objective moral behavior? Where does it come from?

Well, that's a different question to the one you originally asked, and there's two important points to keep in mind here, but one really important thing we need to get out of the way before we can even get to that:

There is no objective morality in EITHER of our positions.

From a purely secular point of view, there is no objective morality; there is, however, an objective framework through which we can develop a situational, context-driven system of morality. You may have heard of it: it's called reality. You can use reality to develop a sense of what's good or bad, because in reality we are beings of a specific nature, who react in predictable ways to stimuli, and for whom that stimuli has specific ramifications; we generally feel pain universally, and it denotes a specific thing for us all, namely bodily damage. Therefore, since pain not only feels bad, but has a specific function biologically that is objectively bad for us, we can determine that causing pain is bad, as its effects are uniformly bad for humans, which we are also, and pragmatically we don't want to be hurt, nor do we want to live in a society that permits that. There are exceptions- vaccinations being an obvious one- but this is a situational ethical scenario, and there is an overriding benefit to vaccinations that renders the temporary pain useful. That's all you really need; a series of at times very basic observations about how we live and interact in reality, and an understanding that occasionally the rules we derive from that may conflict with each other, and that this happens in every ethical system. Morality isn't some incredibly complex thing that's a huge puzzle to figure out without god, it's just a lengthy process to fully encompass, full of ifs and buts and conditional statements. That doesn't mean the benefits of having it aren't obvious, if you take a moment to think about it.

Conversely, from a theistic standpoint there's no objective morality either. You've already asserted that god determines your morality, but god is a subject, by literal definition; if his opinions on morality are what determines its nature, then what you have is a subjective morality that you happen to have imbued with a lot of authority. But that doesn't make it objective, and calling it that inverts the meaning of both those terms, so why even bother using them, at that point?

Either way, the one with the thing closest to objective morality is the atheist, not the theist. But objectivity also doesn't matter, given that neither of us can produce a truly objective morality, since morality doesn't exist as some quantity independent of minds to apprehend it; it's just that I'm not willing to pretend that's otherwise, while theists generally are.