In his important book, The Rise and Triumph of the Modern Self, Dr. Carl Trueman argues that the key idea of our current cultural moment is expressive individualism. The only way to be "true to ourselves," we are told, is to define who we really are psychologically and sexually. This means that our identity is only truly known to ourselves, and others are morally responsible to embrace whoever or whatever we claim about ourselves even if, or especially if, what we reveal contradicts any observable realities.
The Gnostics believed that there is a sharp distinction between the material and spiritual worlds, with the former being evil and the latter good. Since humans are both material and spiritual beings, our physical bodies are evil but our souls are good, possibly even fragments of God that yearn to return to Him. But they can't do this while trapped in our bodies.
Salvation comes through secret knowledge, known as gnosis in Greek, hence Gnosticism. This knowledge differed from group to group. For one group, the secret knowledge was in passwords that would allow adherents to pass through crystal spheres until arriving at the highest heaven, the realm of pure Spirit.
Gnosticism arose in parallel with Christianity. Some Gnostics considered themselves Christian, arguing that the God of the Old Testament was evil since He created the material world. The Father of Jesus, in contrast, was the true God and operated in the realm of spirit without mucking around with the material world. Some of these Gnostics even believed that Jesus was not a true physical being but only appeared to be one, an idea known as docetism.
So, what does all this mean for how we live? Different Gnostic sects had different answers. For some, the body was a hindrance to spiritual development, and so they adopted an austere lifestyle. They might become vegetarians, drink only water rather than wine, and abstain from sexual activity. The last is particularly important since it could lead to babies, trapping another soul into a body. For others, the body was irrelevant to the spirit, and so they would adopt a hedonistic lifestyle, participating in orgies and the like, since these activities don't touch the soul.
Although the details are different from ancient Gnosticism, our culture is awash with Gnostic concepts. It starts with the idea that we need to be true to ourselves, that if we follow the secret knowledge within us we will live a happy and authentic life. External rules about behavior shouldn't hold us back from the things we know in our soul will make us happy. As Woody Allen said to justify his affair with his girlfriend Mia Farrow's daughter, "The heart wants what the heart wants"—and following our heart, that secret knowledge within us, is the advice pop culture consistently drums into children and young adults.
Thus, we follow both sides of ancient Gnosticism: We are sexually promiscuous but anti-natal, since children would hinder our pursuit of our happiness and truth.
Looking within for our truth reaches its logical conclusion in transgenderism, the idea that our true self has nothing to do with our bodies. But, this Neo-Gnostic orthodoxy has nothing in common with God's Word or the reality of His world and the place of our bodies within it.
The story of the Bible is that God created us good, both in body and soul, even if sin has marred both. Our own intuitions about ourselves, and about right and wrong are hopelessly distorted by sin, and so God in His mercy gave us His revelation to tell us about ourselves and to teach us what is good.
Despite the reality of sin all around us, God doesn't make mistakes. Our bodies and souls are matched to each other, and any attempt to fight this will result in more brokenness in our lives. As Ryan T. Anderson put it recently at Wilberforce Weekend, "No one is born in the wrong body, because you are not 'in' a body. You are a body." God's directions for how to live are better for us than the advice of either our fallen desires or our fallen culture. We don't need secret, private truth. God has told us who we are, how we are to live, and united us with Christ who is the Truth to empower us to live in truth.