If your friend is saved....that is to say, he has been born again, then you dont have to worry about his salvation, as its secure in Christ.
Christ hasn't lost one in over 2000 years, and he isnt going to lose your friend.
So, its best to be patient in prayer instead of trying to make him do something., as he'll never be forced.
Here is what you'll learn about people..
When they are stuck in a mindset, this is sort of like them being inside a box with the lid on.
If you try to force them out of the box, all you do is nail the lid down tighter with every effort.
Pray instead that God will give this person a new understanding, as that is the best way and the only quick way your friend will escape his "head" space.
Also, often people who are not satisfied in life, will try something new.
Such is your friend, perhaps.
And this can actually be a good thing, as the more you try that does not work, is one of the best ways to find out what does.
And as only Christ actually works (satisfies), ..... a person who is searching will find their way back sooner or later because as Peter said....."where would i go"?
Finally.
Do you know the pop singer "Sting"?
Use to be the singer in "The Police"..
He was once a communist.
He was a devout communist in England and drove a truck for the movement and delivered their literature.
One day he was thinking......."you know, there is no communist model (country) on the earth that actually works".
"There is no communist country on the earth that supports civil rights, human rights, nor is there a communist country on earth that has created better living conditions for its people then the USA."..
So, at that moment, he realized that communism is a pipe dream, a good idea, that has no model in history that actually works.
Maybe you can tell your friend about Sting.
Just a thought...
K
Yes, i know Sting, but did not know he was a Communist! But he hit the nail on the head with what he said.