Only let us live up to what we have already attained.
Paul is referring to the high call of God in Christ Jesus. A cross-shaped life for sure as we see in the passage itself (click above link). All that is contrary to that is to be considered as nothing more than garbage.
It is wonderful to receive insight and make significant changes. And then it may seem that you’re at a new level, which actually may well be the case. But it is remarkable and disconcerting how almost invariably over time one not only can, but will drift back into the old attitudes or actions.
So Paul’s words here are helpful. They shouldn’t be isolated from their context, but still need to be considered carefully. The emphasis is on how we live. And the context for that is the entire book of Philippians, and for that matter we can say the entire Bible, according to the gospel which is its ultimate focus and fulfillment.
There’s no doubt that once we’ve stepped into a new light and air, it is egregious to us to drift back into the old. We can be sure that all of our advances will be challenged by Satan, and we can say, tested by God. Are we committed to the new course come what may? Or will we yield to the temptation to go back to our old ways?
We need to hold on to what we’ve attained, live up to that. That is part of pursuing the high calling of God in and through Christ Jesus.