As a preface, before I answer your question, let me offer some advice: It would be a good idea to sit down with a Lutheran pastor in your area and ask these questions of him face to face. Any Lutheran pastor worth his call would delight in answering that question for you in detail.
A Lutheran pastor (who holds to the inerrancy of scripture) would share with you that, what Jesus says there, he means. The one who believes in Jesus, in the eyes of God, has crossed over into eternal life. (“ἀλλὰ μεταβέβηκεν ἐκ τοῦ θανάτου εἰς τὴν ζωήν.” (Ἰωάννην 5·24 THGNT-T))
But, in some ways, that only opens up the door to more questions. Is forgiveness a one-time event, or is it a continual event? From a Lutheran (and thoroughly biblical) perspective the answer would be: Both.
In numerous places throughout the Bible, there is this once-for-all emphasis on Jesus' work of atonement. Seven times in the book of Hebrews we have this emphasis that Jesus died once to pay for the sins of all people in the world. Likewise, Paul writes:
2 Corinthians 5:18–19 NIV11-GKE
18 All this is from God, who reconciled us to himself through Christ and gave us the ministry of reconciliation: 19 that God was reconciling the world to himself in Christ, not counting people’s sins against them.
John the Evangelist writes:
1 John 2:2 NIV11-GKE
He is the atoning sacrifice for our sins, and not only for ours but also for the sins of the whole world.
There is the repeated emphasis in God's word that Jesus' sacrifice on the cross happened once. And as a result all people's sins everywhere are forgiven.
But the Bible also lets us know that as often as we confess our sins, Jesus really, truly forgives our sins. John the Evangelist tells us:
1 John 1:8–9 NIV11-GKE
8 If we claim to be without sin, we deceive ourselves and the truth is not in us. 9 If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness.
Which of these is true? Both are. All our sins are paid for. And yet, as often as we confess our sins, Jesus hears our prayers and really, truly forgives them.
This passage from John's gospel is an example of the one-time emphasis on forgiveness. Faith is not a good work that we need to offer up to God to get into heaven. It is not a decision we make. We cannot use our will in conversion, since our will is the very thing that needs to be converted. So God uses his powerful word to create faith in our hearts. But, since God is the one who creates and gives us faith, it is not wimpy and frail. It is not here today and gone tomorrow. It connects us to heaven itself. And that cannot be taken away from us so easily. That is what John is emphasizing in those words from Jesus.