It is clearly, undeniably, here and throughout the Gospel of John, not a future tense verb but present tense. Believing means having eternal life now. I remember when Bishop Werger retired a few years ago. At a farewell gathering of Des Moines pastors he shared that one of his greatest regrets from his many years in ministry was not talking more about the present reality of eternal life. Resurrection changes life now.
New life is a possibility now.
So too is judgment says John. Those who do not believe are condemned already. Perhaps even the teller of this incredible story will be transformed by the telling, and perhaps that transformed life will be the most effective witness of all to the power of God s love for the world. More effective even than holding up a sign at the ball game. Though I suppose that s not a bad place to start.