"What is the main ritual of Christianity?"

Christians have many rituals in their religion but the one I consider the main one is Baptism. Baptism is when a person, typically a child, is welcomed into the Christian lifestyle by having water sprinkled on their head.

"What is the main ritual of Sikhism?"

Sikhs also practise Baptism, just differently than Christians do. They practise Baptism by stirring water in an iron bowl with a double-edged sword while reciting religious hymns, along with an mixing in sugar.