Religion = 'following rules'The word "religion" is used in many ways. A common definition runs something like this, "religion is a set of rules or guidelines or pathways that connect human beings to God." Every "world religion" has a different set of those rules/guidelines/pathways - but all religions share this: to get to God you must follow those rules: religion, boiled down, is about following rules.
Religion is so popular because it answers one of life's Big Questions and does so in a typically human way. How do we connect to God? Religion says: "We can do it, we can connect: just follow the rules."
Christianity is anti-religiousDefined that way, Christianity is emphatically non-religious and even anti-religious. (That's not true of Christianity-gone-to-seed. There are many groups and churches which go by the name "Christian" which are in the end nothing more than religion - over the years they have lost the true essence of the Good News of Jesus Christ and have reverted back to religion - the default mode of human beings, when it comes to God.) True Christianity is about God reaching down to us, not us reaching up to God.
Christianity is non-religious because it answers Life's Big Question in a totally different way to religion. It says that you can't connect to God, you can't do anything. Positive follows Negative: the Good News of Jesus Christ proclaims that God has done everything necessary for us to be restored to God. We can't lift a finger, but God has done everything for us! Through his Son he has reached down to us.
The Gospel teaches that in this reconnection, a new relationship with God is restored. God does not remain aloof and distant, he becomes our Father in heaven and we become his children. But it is God who does it all, not us.
Religion is spelt DO and Christianity is spelt DONE.
Religion is spelt RULES and Christianity is spelt RELATIONSHIP.
What's the real problem with religion?Sin. The reason we can't either get right or stay right with God is because of our sin. The reason rules don't work is that no-one can keep them. This is the essential problem of mankind - our sin. If we could perform, if we could be good, religion might work, but we can't. And this is why God has to take both the initiative - and the continuing care.
The Gospel of GodAccording to the Gospel of Jesus Christ a sinner is declared righteous the moment they believe in Jesus Christ! That righteousness does not come through our obedience. It comes through Christ's earthly obedience reckoned to our account and our sins imputed to his account - and paid for on the cross. Instantly we are justified by God, through faith. Once converted we are then sanctified. Again, God is at work in us to sanctify us by Word and Spirit (though in sanctification we must work with him, in fear and trembling).
The problem with religion is our sin which makes us completely unable to 'perform' spiritually. The Gospel is God's solution to that problem of sin - it's a Gospel of grace, God's initiative and ongoing activity towards undeserving sinners to do what we cannot do ourselves.