ALTHOUGH GOD ASSIGNS our biological sex at conception, masculinity is learned behavior. That means that you can have a man's body but be feminine in behavior and orientation. Likewise, you can have a woman's body and be thoroughly masculine in your approach to life.
This is a problem because the Bible never defines masculinity. It just assumes it. In the ancient world there was little gender confusion, nor could anyone anticipate a day when it would arrive.
Because the Bible doesn't address it directly several notable authors have taken a stab at defining biblical masculinity.
In his book Church Planting Is For Wimps, author Mike McKinley writes, “Being a real man means being responsible, dependable, humble, and strong. It means pouring yourself out for your wife and kids. It means walking closely with Christ and taking care of people in need.”
Dr Leonard Sax says it this way. “What does it mean to be a man?” The answer is: being a man means using your strength in the service of others.”
On the other hand, Doug Wilson writes, “Masculinity is the glad, sacrificial assumption of responsibility.”
Last, John Piper writes, “At the heart of mature masculinity is a sense of benevolent responsibility to lead, provide for, and protect women [and children] in ways appropriate to a man’s differing relationships.”
These definitions all share several ideas in common. First, they see masculinity as something moral, not physical. Second, they center it in words like initiation, assuming responsibility, serving, providing for, sacrificial living, etc.
So where should Christians go to get a solid idea of what masculinity looks like? The Bible doesn't tell us about masculinity: it shows us a picture of masculinity. Jesus was the Second Adam. He was everything that God designed the first Adam to be. Jesus was the ideal man. He modeled masculinity as God intended it. Therefore, a good place to see it fleshed out is Paul's short summary of his life and ministry in Philippians 2:5-8.
“Have this mind among yourselves, which is yours in Christ Jesus, who, though he was in the form of God, did not count equality with God a thing to be grasped, but emptied himself, by taking the form of a servant, being born in the likeness of men. And being found in human form, he humbled himself by becoming obedient to the point of death, even death on a cross" (Philippians 2:5–8).
In short, Christ-centered men are most app to express masculinity as God intended it. And we see Christ through the gospel. Therefore, Gospel-centered men are most apt to be Christ-centered. And Christ-centered men are most apt to be masculine.
 Mike McKinley, Church Planting is for Wimps, (Wheaton: Crossway, 2010) pg 102
 Leonard Sax, Boys Adrift (New York: Basic Books, 2009 Kindle Edition). p. 181
 Grudem, Wayne (2006-08-31). Recovering Biblical Manhood & Womanhood: A Response to Evangelical Feminism (pp. 5-6). Good News Publishers/Crossway Books. Kindle Edition.