These things I have spoken, and am still speaking, to you (perfect, not aorist) with this purpose, that the joy that is mine may be £ in you. This is variously explained. Augustine, "My joyfulness concerning you," which is scarcely the burden of the previous verses; Grotius, "Your delight in me," which would be somewhat tautologous; Calvin and De Wette, "The joyfulness capable of being produced in you by me, might be in you."
But the words are more simply explained by Lange, Meyer, Lucke, Westcott, Alford, and Moulton, as the communication to his disciples of his own absolute and personal joy. "The joy that is mine," like "the peace which is mine," is graciously bestowed.
A joy was set before him, the joy of perfect self-sacrifice, which gave to his present acts an intensity and fullness of bliss. It was this, in its motives and character and supernatural sweetness, which would be in them.
If they receive his life into them, it will convey not only his peace, but that peace uprising and bursting into joy; and he adds, in order that your joy may be fulfilled, i.e. perfected, reach its highest expression, its fullness of contents and entire sufficiency for all needs.
1 John 1:1-4 is the best commentary on this last clause. The Old Testament prophets had often spoken of Jehovah's joy in his people, comparing it to the bridegroom's joy, and the bride's (Isaiah 62:5; Zephaniah 3:17).
This entire idea is linked with 1 John 1:10; where the keeping of his commandments, from motives of love, will enable the disciples to "abide in his love." He now passes the whole law of the second table into the light of his joy and the power of his example.