A quick answer is that it is a form of religious belief. That’s obvious. But much more needs to be said.
Basically there wouldn’t be a religious belief called ‘Christianity’ if someone called Jesus hadn’t lived. To put it in easy terms, Jesus was a person whose life, both in what he did and what he said, showed and told us what God needed.
Jesus taught and lived a moral way of life, he healed some people who were ill and he forgave others who’d done wrong.
He challenged evil in any way it came to him in very forthright terms. He showed us, and this is really important, that beyond our present lives there is a new life close to God.
The sum total of this is that God loves us and in Jesus showed us what this love means.
However Jesus’ own personal story didn’t have a good ending for him. He was killed after arrest, a summary trial and swiftly enacted execution.
But even though he died, his promise was that after he’d gone, the will of God to carry on doing the things he had done would move into ordinary people like us.
Jesus promised us that because of this we’d be able to do far more than even he had done. This ‘will of God’ is what we call the Holy Spirit of God.
Christianity, holds to the fundamental belief that God has always existed in some way beyond time, but that God has come into human life in history through Jesus.
This happened as God became a human being in Jesus, even though still remaining God at the same time. And God continues to come into us as we seek to respond to God’s leading in our lives through the gifts and skills given us by the Holy Spirit.
God loves us and wants us to respond to the love God has for us by our care for one another as well as by the prayers we pray and by the worship we share.