[I'm writing this as a response to a call from Rachel Held Evans on her blog. I know that I am not exactly being true to her instructions to "Write a blog post that highlights the feminine images of God found in Scripture or that celebrates the importance of women in the Church. (Be positive and be creative!)," but this is what I know so I thought I'd give it a shot.]
We make a fundamental(ist) error when we forget that religion is really about myth-making. That may seem like a very dismissive statement, but it is actually quite the opposite. What the sociology of religion has shown is that the way that societies think about God is really the way that they think about themselves. In other words, our conceptions of the divine are really just a reflection of our own social organization and cultural values (cf. Matthew 25:31-46). After all, (that thing that we call) God is, by definition, ineffable so anything we "eff" about God just points in the general direction--we hope--of what we cannot possibly fathom. In the absence of an alternative way to communicate the incommunicable, we search for metaphors. This is where myths come in. Myth, contrary to the sense of "false" that the word usually carries today, actually is based in profound truth. Take for example the myth of King Oedipus. Forgetting all the mother-loving stuff that Freud obsessed over, one of its essentially themes is about the conflicted relationship between fathers and sons. Most of the Ancients did not understand the tale of Oedipus literally; they saw it for what it was, an allegory pointing to a deeply held truth.
So, what does this tell us about Piper's assertions about a masculine Christianity? Let's assume for a second that Piper is right that the most of the collection of books in what we call the Bible paint God in masculine language. Does that tell us more about God or about the men who wrote the Bible? Clearly, since God transcends sex and gender--after all these are biological and social distinctions respectively--it tells us more about ourselves, the human beings who wrote, retold, collected, shared, and continually reinterpret the Bible. All of the books of the Bible were written in deeply patriarchal societies, and that patriarchal deference for the masculine is reflected in the text. Here, Piper makes that fundamental(ist) error, mistaking the metaphor for the reality.
Today, however, we are slowly moving beyond patriarchy (although we're not quite past it, yet). We value the feminine much, much more than we did in the past. As we do, we must confront the legacy of our myths and search for the myths that do in fact reflect not God, but ourselves. In doing so, we ironically get closer to God.