You are viewing the verse as though it said, "that he which hath begun a good work
in you [singular] will perform it until..."
Here's how I see the text (according to the Grk) [speaking of "the Church WHICH IS HIS BODY" (see again
Eph1:20-23 WHEN [as to its existence])]:
Philippians 1 -
"
5 in view of your participation in the gospel
from the first day until now.
6 For I am confident of this very thing, that
He who began a good work [SINGULAR] in you [PLURAL, corporate 'you'] will perfect it [SINGULAR] until the day of Christ Jesus. 7 For it is only right for me to feel this way about you all, because I have you in my heart, since both in my imprisonment and in the defense and confirmation of the gospel, you all are
partakers of grace with me.
8 For God is my witness, how I long for you all with the affection of Christ Jesus.
9 And this I pray, that your love may abound still more and more in real knowledge and all discernment,
10 so that you may approve [/put to the test, prove, examine; distinguish by testing]
the things that are excellent [/that differ],
in order to be sincere and blameless
until the day of Christ;
11 having been filled with the fruit of righteousness
which is through [/by means of] Jesus Christ, to the glory and praise of God."
You are thinking "you, INDIVIDUALLY" rather than "you, CORPORATELY" (as part of the "ONE BODY" that will be "caught up TOGETHER [at the same time / 'at once'] with [in union with, G4862]" the dead "IN Christ").
Same here:
2Cor11:2 -
"For I [Paul] am jealous as to
you [plural, corporate 'you'] with
the jealousy of God.
For I have betrothed you [plural, corporate 'you'] TO ONE HUSBAND,
to present A CHASTE VIRGIN [SINGULAR] to Christ."