Christian: a person who believes that the only way to connect with the only God of the universe is to trust in the person of Jesus alone — the unique, fully God, fully human son of God — to pay for the debt of their sins, and therefore submits wholeheartedly to Him and to the pursuit of following Him.
Good Person: somebody who does their best not to hurt others and enacts good will toward others; good moral character (aka "a person who fears God").
These definitions are how I understand these terms, but there are a thousand other ways to define them—whole books written on them, entire degrees gained by researching them. When I hear self-identifying Christians talk about these two terms, I often hear them used interchangeably, though in my mind they focus on different aspects of a person's life. "Christian" describes belief and tranformation dependent of that belief. "Good person" describes an assimilation to relative cultural values.
Sometimes these aspects overlap. Sometimes they don't. In fact, I've met plenty of culturally good people who were not Christians. And Christians who were not, in a given moment, good people.
I think part of this confusion is that to be a Christian means to be in the process of transformation. So depending on at what point you jump in on their story, it may be hard for to you to see the effects of Jesus.
Unfortunately I have also met people who think of these two terms as opposites! This makes me particularly sad, as my personal understanding of Jesus is that He is the ultimate illustration of a good person and has every right to be revealed as such. It's a terrible shame to be a people who claim His name while demonstrating the opposite of His character.
But I think the real miss is to assume that just because you are a good person that you are also a Christian. They simply are not the same. In fact, being a good person may, indeed, cost less than surrendering your whole life, your pride, and your glory to something bigger than you.
Unsplash photo cred: Ivan Bandura