I tell you the truth, he who believes has everlasting life.
Believes what? In the context it is he who believes Jesus's claims. This is later confirmed to the disciples when Thomas fails to believe unless he sees. Blessed are those who have not seen and yet have believed. This applies to us. This is Holy Spirit given faith.
Everlasting life begins with faith and continues forever - not as a reward but as a reality. Living with the Spirit is sharing in His accomplishments and vitality.