And we write these things to you, that our joy may be fulfilled.
There is abundant testimony in the New Testament to the effect that not all of those qualities of eternal life ultimately expected are in the possession of the saints now.  Even the apostle John's joy was not yet full when he wrote this epistle, as indicated by the last verse of the prologue (1 John 1:4) above.  Paul's statement that it would be "better" to depart and be with Christ; John's declaration that "it is not yet made manifest what we shall be" (1 John 3:2); the absolute inability of any Christian ever to rise completely above all sin; the fact that it has never even entered into the heart of man  (1 Corinthians 2:9) the things that God prepared for them that love him; the constant attendance upon human footsteps of sorrow, pain, and tribulations; - all such considerations deny the quality of that eternal life in Christians now as having any complete correspondence to the eternal life given on the last day to them who shall be invited to "enter thou into the joy of thy Lord"  (Matthew 25:23).  "Entering in" cannot be equated in any complete sense with "You have already entered."