In the context in which the text appears, I would have to say that "TWI"
refers to God the Father. Look at the block beginning with EPH 3:14 and
ending with 21. This is a unified, Trinitarian prayer.
"For this reason, I kneel before the FATHER, from whom his whole family
in heaven and earth derives its name. I pray that out of His glorious
riches He may strengthen you with power through His SPIRIT in your inner
being,
(NOTE: the connection between the Father and power. The only noun which
could be the antecedent of "His riches" and "He may strengthen you with
power" is the Father.)
...so that CHRIST may swell in your hearts through faith. And I pray
that you, being rooted and established in love, may have power, together
with all the saints, to grasp how wide and long and high and deep is the
love of CHRIST, and to know this love that surpasses knowledge- that you
may be filled to the measure of all the fullness of GOD.
"Now to Him who is able to do immeasurably more than all we ask or
imagine according to His power ...
(NOTE: Power was mentioned before, directly associated with the FATHER,
so Paul is probably referring to the FATHER again, here)
... that is at work within us, to Him be glory in the church...
(Probably refers to the Spirit, as the activity of the church was
evidence of the presence of the Holy Spirit. For example, notice in the
Apostles Creed - the oldest summary of the Christian faith - in the
paragraph about the father, his nature as creator is mentioned. In the
paragraph about Jesus Christ, his life, death and resurrection are
mentioned. In the paragraph about the Holy Spirit, the Church is
mentioned.)
...and in CHRIST JESUS throughout all generations, for ever and ever!
Amen. (NIV)
(Obviously refers to the Son.)
So you see, this is a profound Trinitarian statement, closing a
Trininarian prayer on behalf of the Ephesians.