This is a huge topic on which much can be said. To over-simplify tremendously, I'd offer three points:
- The life, death resurrection of Jesus
- The extraordinary growth of the Church, from a persecuted minority to taking over the Roman Empire to the massive worldwide following she has today, not least in countries like S Korea and China.
- Philosophically: if God exists He is obviously not incompetent. So one of the major purported communications from God to humanity is likely to be genuine, probably the one with the largest number of adherents spread over all the world, who are not kept in line by threats of violence.