doubt over salutation

At the beginning of a letter, the salutation may be formal or informal. If informal, then Dear is followed by the first name - eg "Dear Samuel," whereas if it's formal then you use the title and the surname - eg "Dear Mr Johnson," Dear Ms Evans", "Dear Dr Roberts".

English differs from some other languages in that you never....
a) use the first name between the title and surname - eg * Dr Mr Samuel Johnson
b) use the title with the first name only - eg *Dear Mr Samuel
c) use a professional title (with the exception of Dr) - eg *Dear Engineer Brown or *Dear Architect

There's nothing here to really "understand" - it's just convention. Each language has its own conventions for acceptable salutation forms. You just need to find out what they are, and follow them.

DOES IT HAVE something to do with the CHRISTIAN NAME?

I thought I'd answered this, but the reply seems to have disappeared. I don't understand the question? Does what have something to do with the first name?? If you're thinking that it might depend on eg whether it's a male or female first name, or a long or a short one, or whatever - no, it makes no diffeence. The rules are as above.

Sorry, I did not make myself understood.

I just wanted to know whether the reason is 'we do not use the Christian name in the salutation part'

I had this doubt because my teacher told me this, but I did not try to understand WHY, for I had known language as a barbarous thing.Thank you very much for your answer .

"Christian name" is just another expression for "first name" KCChan - but less usual nowadays. Use "first name".