All people have a relationship with God.

It is common for church people to say Christianity is a relationship.

Christianity involves relationships, but that is not all it is.

Also, it is a vague statement, that needs to be explicit. Some people are related to God as His children. Others are in relationship with God as Judge and condemned. Still others are in a relationship with God as Spitter and spit, so to speak.

Revelation 3:16 KJV

[16] So then because thou art lukewarm, and neither cold nor hot, I will spue thee out of my mouth.

Probably, people who say Christianity is a relationship have something specific in mind. It would be wonderful if they would say what that is.