What about secular humanism? No threat whatsoever.
A materialistic/scientific worldview? Not a threat to Christianity.
Surely socialism is the greatest threat to Christianity! No it is not.
How about Communism? No.
How about evil dictators? No.
Not even corrupt constitutional republican democracies are a threat to Christianity.
If you think that any of these are grave threats to Christianity, then you may have too-low view of Christ's creative capabilities to further his redemptive work in the world.
However, as one observes the world, particularly America, there are two things that currently seem to be the greatest global threat to Christianity. Political power and material wealth. Christ was regularly rejected the most by those with vested interest in either maintaining or gaining political power. Actually, they ended up killing him because he wouldn't shut up. And Jesus went out of his way to point out the obstacle that material wealth would become to those who wanted to follow his Way.
World history will usually show that where there are great centers of political power, Christianity will quickly be co-opted and become a force for oppression and reinforcement elitist rules. And where Christianity becomes a tool for accumulating material wealth, that kind of Christianity will become impotent and useless to resist injustice towards widows and mercy towards sinners.
In light of what Jesus says about himself, about his Way, about his Work in his world, he kind of goes out of his way to emphasize the role of servanthood and salvation, blessing and binding up the wounds.
Jesus knew all about hardwork, being a craftsman he was familiar with the daily exertion of shaping stone, of carving wood, of chiseling marble, of drawing the plumb line. He worked in collaboration with his family to care for each other and their neighbors. And when he walked away from his life as a craftsman to fulfill his role as a prophet - he drew around him people of wealth who were inspired to become generous to the poor, the widow, the crippled, the prisoner. At one point gives assurance: you'll know heaven is your home, you'll know you trust me for salvation if you're giving fresh water to the weary.
So, for all the Christians in America who live in fear of indoctrination, of fear that socialism will ruin our country, in fear of what gay-marriage will do to the institution of marriage, in fear of how we are losing our way morally. I say this: Fear Not. God is still with us.
Jesus is present - the Same Spirit that came upon Him in his day is the Same Spirit that is upon us in our day. It matters not how politically corrupt our country is - it is no obstacle for Christianity unless Christianity has wedded itself to political power as a means to further its work. Material wealth is no obstacle for Christianity unless Christianity has become a means to attain more material wealth at the expense of being a servant of God like Jesus, a healer like Jesus, a rescuer like Jesus, a wisdom-teacher like Jesus, a rebel like Jesus, a kingdom-come kind of believer like Jesus.