The passage you need, and it is my favourite, is 1 John 5:11-12.
"And this is the testimony: God has given us eternal life, and this life is in his Son. He who has the Son has life; he who does not have the Son of God does not have life." (NIV)
Notice that it is written in the present tense. When you read the context, including the first chapter, you will see that Eternal Life resides innately within Jesus Christ, and that a believer shares in that Life. When a person stops believing in Jesus, they no longer share in that Life. It is thus possible for a person to have Eternal Life here and now, yet lose it.
Eternal Life speaks of its qualities, not its length.
How do we know? Not by feeling, not by emotion.
A most helpful reference is "Life in the Son", by Robert Shank, pages 301 to 305. Do you have access to the book?
Doug