“believes” = “believeth on” (KJV) = pisteuo = to believe to be true so that one places confidence in it; to be persuaded and commit trust to. Just believing in Jesus (giving mental assent to His existence and state of being) does not bring eternal life; we must believe so fully that we give ourselves wholly to Him, completely trusting Him to do the best for us.

“has” = present tense. Eternal life begins when we place our trust completely in Jesus.