Use 'believe in' when affirming your conviction about something's existence or its truth, even without proof. Many people believe in aliens, despite the lack of evidence.
Pro Tip 2/3
Personal Convictions
It often expresses personal convictions that might not be universally held. Some believe in the power of dreams to predict the future.
Pro Tip 3/3
Cultural Sensitivity
Be aware of cultural diversity when discussing what you or others believe in. In some cultures, they believe in spiritual healers.