If I don't know means "I believe god exists but I won't confirm" then yes, they can be christian. Same as if they said "I believe in god even though I have my doubts" one can be christian.
Someone said to me once, "why don't you believe just in case." If salvation is more about belief or faith (which can be agnostic position-I trust something exist but I can't prove it does objectively), then yes. One can be agnostic and christian. If one doesn't believe god exists because he or she doesn't know, then no, that's a contradiction.
If I remember correctly: blessed are those who believed but have not seen. So, by definition, if you haven't seen god to know he exists by your earthly senses, it would make sense to say one doesn't know. Though many agnostics don't follow christianity because they don't know rather than follow christianity despite they can't know everything.