If you have not read Stories Versus Truth, you may want to read that before reading this.

We say this to explain that you can answer this question for someone, yet, they will only receive it if they have a will to know the truth. They can know the truth. Jesus said, "If you really want to obey God, you will know if what I teach comes from God or from me." He also said, "You will know the truth, and the truth will set you free" (John 8:32) "When we love others, we know that we belong to the truth, and we feel at ease in the presence of God" (1 John 3:19) God is love. The real problem is right here: "If you really want to obey God..."

Debates divide.  Don't let Satan turn a discussion into a debate.  Look at the other person's point.  There is likely to be some measure of truth.  But you can see the obvious error in thinking.  How are they defining religion?  Do they think that their own stand is outside of religion?  Do they think that their own stand doesn't divide?

We know by revelation. Revelation comes both through the physical, material world and through the spiritual realm. (Find out more about revelation and why it is the only way to really know anything with certainty)  

If we define religion as something that people do, then we can go on from there and say that division exists fundamentally and religion is part of the outworking of that division. God is not dependent on human religion.  The division is between God and His followers and Satan and his followers. There is a war going on in the spiritual realms, and that war extends to the earth. "Think not that I came to send peace on the earth: I came not to send peace, but a sword. For I came to set a man at variance against his father, and the daughter against her mother, and the daughter in law against her mother in law" Matthew 10:34-35 Christ knew that this war was going to tear things up here on earth. The violence against Christians by other religions has increased dramatically in the last 100 years, particularly from Atheists, Muslims, and Hindus in that order. Sometimes, Christians have fought back, but such fighting is against the teachings of Christ Who said, "Turn the other cheek." Christ predicted that they would kill us, but told us to rejoice in persecutions. (see also)

Christ came to reconcile all people and eventually the whole creation back to God, yet only a few would have it. The rest have made themselves enemies of Christ. Some of these enemies have become violent.

Some people may bring up the Holy Roman Empire as if that were the Church or as if that empire in any way followed the teachings of Christ. There is a reason that those years are called the dark ages. The Church rejected the leading of God and went the way of the human. They became persecutors of any Christians who were following Christ. Everyone who calls himself or herself a Christian is not always following Christ. Some never follow Christ. At this point, I would doubt if anyone always follows Christ. We do have Christ living within, but we live far below our calling most of the time. This is a progressive walk from glory to glory and from light to light, so we can expect that immaturity eventually becomes maturity if we continue in submission to the Holy Spirit's leading.