Matt 28.19 Go therefore and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit,
To whom exactly is this command given? Not the new believers.
If it is disobedience to not be baptized, it is disobedience on the part of the person sharing the gospel, NOT the new believer.
And Jesus came and spake unto them, saying, All power is given unto me in heaven and in earth. Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost: Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you always, even unto the end of the world. Amen. - Matthew 28:18-20 ..you will note in there he says "teaching them " it must be taught (preached)what they are to do.
Then it's also repeated to teach "them" to observe (do) all he commands.
So it's very plain it is to be taught and then the person is to respond obediently .
And else where the list is more complete saying to heal the sick ,preach the gospel,drive out devils,..baptizing them.. Baptism is an obedient response to the instructions of the Gospel being taught.disobey if we don't teach it.they disobey if they don't respond .since repentance is the first instruction ,baptizing a person outside their will and without their repentance is nothing more then a forced swim.
And the Op asks "can They be christian if they are not baptised".(not are they saved)not all Christians are saved.Jesus says so in the parables) so if they say they are christian and then are taught about baptism and then refuse to obey Jesus teaching.they are in a state of Un repentance. They are going "their own "way .they need to repent( rethink change there mind )and go God's way
My reply is ..untill they Do they cannot be his disciples.