What should we do if someone attacks our beliefs and criticizes the Dharma?
Each situation must be regarded individually. In general, if we feel, "They are criticizing my beliefs. They think I am stupid for believing that," we are clinging to our beliefs because we think, "These beliefs are good because they are mine. If someone criticizes them, they are criticizing me." Such an attitude isn't very productive and we'll be more peaceful if we abandon it. We are not our beliefs. If others disagree with our beliefs, it does not mean we are stupid.
Being open to what others say is useful. Let's not be attached to the name and label of our religion. We are seeking truth and happiness, not promotion of a religion because it happens to be ours. In addition, questioning the teachings is reasonable. The Buddha himself said we should check his teachings and not just believe in them blindly.
On the other hand, we should not automatically agree with everything someone else says. We should not abandon our beliefs and adopt theirs indiscriminately. If someone asks a question we cannot answer, it doesn't mean the Buddha's teachings are wrong. It simply means we don't know the answer and need to learn and contemplate more. We can then take the question to knowledgeable Buddhists and think about their answers. When others question our beliefs, they are actually helping us deepen our understanding of the Buddha's teachings by showing us what we do not yet understand. This inspires us to study the Dharma and reflect on its meaning.