First of all, I think that your friend makes this claim then he should be the one to also provide the book, chapter and verse to back it up (rather than simply tell you that it is found “in many places” in the Bible and leave it at that).
It has been my observation that when Evangelicals elaborate on the concept of accepting Jesus as ones personal Lord and Savior, the following Bible verses are mentioned:
“If you confess with your lips that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved.” (Rom 10:9)
“I have been crucified with Christ; it is no longer I who live, but Christ who lives in me; and the life I now live in the flesh I live by faith in the Son of God, who loved me and gave himself for me.” (Gal 2:20)
From Romans 10:9 we can see the reference to Jesus as “Lord,” and if he saves us then he is the “Savior.” In Galatians, Paul expresses the actions of Jesus in a personal way: “…who loved me and gave himself for me.”
When did “accepting Jesus as personal Lord and savior” become the mark of “being a Christian” or has it always been a requirement?
That’s a good question. Even though the terminology can be, more or less, drawn from the NT, the phrase “accept Jesus as your personal Lord and Savior” is so standardized that it seems like someone must have coined this phrase at some point, and it caught on and spread. Awhile ago my brother attempted a thread to discover who coined the term, but nothing was determined.