John is the only book in the Bible that claims it was written for the express purpose of bringing people to eternal salvation. Its purpose statement in John 20:30-31 says,

And truly Jesus did many other signs in the presence of His disciples, which are not written in this book; but these are written that you may believe that Jesus is the Christ, the Son of God, and that believing you may have life in His name.

Therefore, we should begin with John to understand how to be saved and examine it closely to discover the condition for salvation.

How the condition is presented

1. The purpose statement itself says if we believe we have eternal life.
2. The verb believe is used 98 times in John as the condition for salvation. This compares to only 34 times in the other Gospels and 116 times in the rest of the New Testament.
3. The consistent result of believing is salvation. When one believes, one can know that he/she has eternal life as a present possession (John 5:24; 9:38).
4. The figures of speech used to picture belief in John (receive, look, hear, enter, feed, come) denote receptivity, agreement, or trust. All are essentially simple or passive activities. None communicates the idea of merit, work, effort, or achievement.

How the condition is not presented

1. There are no qualifiers used with believe, such as really believe, truly believe, or genuinely believe. There is only one kind of belief.
2. There are no other conditions mentioned in John, such as repentance, surrender, commitment, or obedience.

Conclusion
We are saved when we simply believe (trust in, accept as true for myself) that Jesus is God’s Son, the Savior from our sins. Salvation is absolutely free and apart from any condition of merit to earn it.