What is the biggest threat to Christianity?

This might come as a surprise to some, but the biggest threat to Christianity exists inside the church, not outside the church. According to the Bible, the biggest threat to Christianity is legalism.
Since the fall of man in Genesis 3, the human race has been naturally prone toward works-righteousness, self-salvation projects. Having determined way back then that we could do it better on our own, we’ve been trying ever since.
There’s a common misunderstanding in today’s church, which says there are two equal dangers Christians must avoid. On one side of the road is a ditch called “legalism”; on the other is a ditch called “license” or “lawlessness.” Legalism, they say, happens when you focus too much on law, on rules. Lawlessness, they say, happens when you focus too much on grace. Therefore, in order to maintain spiritual equilibrium, you have to balance law and grace. If you start getting too much law, you need to balance it with grace. If you start getting too much grace, you need to balance it with law. This dichotomy exposes our failure to understand gospel grace as it really is; it betrays our blindness to all the radical depth and beauty of grace.
It’s much more theologically accurate to say that there is one primary enemy of the gospel—legalism—but it comes in two forms. Some people avoid the gospel and try to “save” themselves by keeping the rules, doing what they’re told, maintaining the standards, and so on (I call this “front-door legalism”). Other people avoid the gospel and try to “save” themselves by breaking the rules, doing whatever they want, developing their own autonomous standards, and so on (“back-door legalism”). In other words, there are two “laws” we can choose to live by apart from Christ: the law which says, “I can find freedom and fullness of life if I keep the rules,” and the law which says, “I can find freedom and fullness of life if I break the rules.” Either way, you’re trying to “save” yourself, which means both are legalistic because both are self-salvation projects. So what some call “license” is just another form of legalism.
This distinction is super important because the biggest lie about grace that Satan wants the church to buy is the idea that it’s dangerous and therefore needs to be kept it in check. The perceived fear is this: if we think too much and talk too much about grace and the radical freedom it brings, we’ll go off the deep end with it. We’ll abuse it. By believing that lie, we not only prove we don’t understand grace, but we violate gospel advancement in our lives and in the church by perpetuating our own slavery. The truth is, disobedience happens not when we think too much of grace, but when we think too little of it.
As a pastor, one of my responsibilities is to disciple people into a deeper understanding of obedience—teaching them to say no to the things God hates and yes to the things God loves. All too often I’ve wrongly concluded that the only way to keep licentious people in line is to give them more rules—to lay down the law. The fact is, however, the only way licentious people start to obey is when they get a taste of God’s radical, unconditional acceptance of sinners. What licentious people need is a greater understanding of grace, not a governor on grace. Grace alone melts hearts and changes us from the inside out. Progress in obedience happens only when our hearts realize that God’s love for us does not depend on our progress in obedience.
A “yes, grace—but” disposition is the kind of fearful posture that keeps legalism swirling around in our hearts and in the church.
Tullian Tchividjian