dc87,
You sound just like me forty years ago when I asked the same questions, but the internet was not an option back then so I talked to people who were Christians. I was raised in church where I heard the Gospel just about every Sunday, but for some reason I never fully understood the “believe in Jesus” part of salvation. Consequently, as a young adult, I found myself doubting my “salvation” at age 13. Like you I questioned the scriptures that stated that the devil believes in Jesus. Another thing that was confusing to me was that if the bible is true then the devil knows that Jesus is the Son of God, and the devil cannot be saved, but I had heard people say that they are Christians because they believe that Jesus is the Son of God. So, the most confusing part about “believing in Jesus” was the meaning of “believing.”
Below is a link to my testimony if you would be interested in reading how I found the real meaning of “believe in Jesus.”
My testimony
Just in case you don't want to read a lengthy testimony I'll explain the truth I found about “believing” and how I found that truth.
I had never read anything in the bible, except at church, and that was only because we were asked to turn to the book of X chapter xx verse x, and honestly back then my mind was not thinking about God. But, now that I WAS concerned about my relationship with God, one night I sat down at the kitchen table, opened a bible to the Gospel of John, and began reading in John 1:1. As I read there the Holy Spirit opened my spiritually blind eyes and allowed me to understand the truth found there. Now I will admit that I don't pretend to understand everything about the bible, God, and Jesus, but I did find the answer about “believing in Jesus” as I read in the Gospel of John.
The Holy Spirit showed me that I was looking at myself, at the things “I” was doing instead of looking at what Jesus had already done. He showed me that my faith was in “my faith.” As I read through the Gospel of John the Holy Spirit changed my focus from “self” and pointed me to the finished work of Jesus on the cross. To make this short & simple I'll just say that He, the Holy Spirit, convinced me that when Jesus went to the cross He took my sins with Him there. The Holy Spirit totally & completely convinced me that when just before Jesus died He said “it is finished” that Jesus was saying He had just paid my sin debt to God, “paid in full.” Being convinced by the Holy Spirit of the matter is the same as “knowing.”
Based on my personal experience I would define believing in Jesus as being convinced by the Holy Spirit, through the Gospel message, of the truth that Jesus took care of my sin problem when He died on the cross & shed His blood there, and as a result of being convinced of that truth I was moved in my spirit by a overwhelming sense of relief resulting in an immediate transfer of confidence from self to the finished work of Jesus on the cross. I would also define being “born again” in the same way, because the process involves the Holy Spirit giving the new believer the new life in Christ.
John