How can we know for sure we have eternal life?

Did you know that the Bible tells us that we can know for sure that we have eternal life? Check out these sample verses:

1 John 5:13
I write these things to you who believe in the name of the Son of God so that you may know that you have eternal life.

John 20:31
But these are written that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name.

Ephesians 2:8-9
For it is by grace you have been saved, through faith--and this not from yourselves, it is the gift of God-- not by works, so that no one can boast.

Romans 10:13
for, "Everyone who calls on the name of the Lord will be saved." (NOTE: This verse is also in Acts 2:21 and Joel 2:32.)