That is (touto de estin). "An explanatory correction" (Denney). The de should not be ignored. Instead of saying that he had a spiritual gift for them, he wishes to add that they also have one for him. That I with you may be comforted (sunparaklhqhnai en umin). "My being comforted in you (en umin) together (sun-) with you," a mutual blessing to each party (you and me).