{God is faithful} (pistos ho theos). This is the ground of
Paul's confidence as he loves to say ( 1Th 5:24  1Co 10:13  Ro
8:36  Php 1:16 ). God will do what he has promised. {Through
whom} (di' hou). God is the agent (di') of their call as in
 Ro 11:36  and also the ground or reason for their call (di'
hon) in  Heb 2:10 . {Into the fellowship} (eis koin(936e)ian). Old
word from koin(936e)os, partner for partnership, participation as
here and  2Co 13:13f.  Php 2:1  3:10 . Then it means fellowship
or intimacy as in  Ac 2:42  Ga 2:9  2Co 6:14  1Jo 1:3,7 . And
particularly as shown by contribution as in  2Co 8:4  9:13  Php
1:5 . It is high fellowship with Christ both here and hereafter.