Philippians 1:6
“Being CONFIDENT of this very thing, that he that hath begun a good work in you will perform it until the day of Jesus Christ.”
It is the Holy Spirit working in us and will continue to work in us until the day of Christ.
He guides us, teaches us and perfects us unto a perfect man. He is there in us the moment we are saved. This is His job for the members of the Body of Christ. Sealing us and spiritually baptizing believers into the Body of Christ, the One New Man
And we can have every confident this is being performed on every believer.
Agreed, but we must cooperate with that Spirit. That God sends us his Spirit does not give us license to return to a life of sin. This is clear in Paul in the passage just quoted and elsewhere, e.g.:
For you were called for freedom, brothers. But do not use this freedom as an opportunity for the flesh; rather, serve one another through love. For the whole law is fulfilled in one statement, namely, “You shall love your neighbor as yourself.” But if you go on biting and devouring one another, beware that you are not consumed by one another. I say, then: live by the Spirit and you will certainly not gratify the desire of the flesh. For the flesh has desires against the Spirit, and the Spirit against the flesh; these are opposed to each other, so that you may not do what you want. But if you are guided by the Spirit, you are not under the law. Now the works of the flesh are obvious: immorality, impurity, licentiousness, idolatry, sorcery, hatreds, rivalry, jealousy, outbursts of fury, acts of selfishness, dissensions, factions, occasions of envy, drinking bouts, orgies, and the like. I warn you, as I warned you before, that those who do such things will not inherit the kingdom of God. In contrast, the fruit of the Spirit is love, joy, peace, patience, kindness, generosity, faithfulness, gentleness, self-control. Against such there is no law. Now those who belong to Christ [Jesus] have crucified their flesh with its passions and desires. If we live in the Spirit, let us also follow the Spirit. Let us not be conceited, provoking one another, envious of one another. (Galatians 5:13-14)
It seems like there is a difference of opinion of just when this 'day' is.