Paul exhorts in Philippians to "work out your salvation with fear and trembling."
We are not saved by works. However, walking in salvation provided by God is a pathway, the pathway of Jesus.
We walk the path by learning to walk in the Spirit of God. This is knowing Jesus as the way. We pass through this life in the way of Jesus and it is a work in progress. We "work" to sort our selfish desires of the natural man from the pure and righteous desires of the Spirit of God living within.
It's a work because of learning the depth of self-denial even unto the death of the natural man, which we find emotionally painful.
It is work because it is a daily-- no a moment to moment --discipline.
Think of it this way: Our Father is full of resource available in the Spirit for us, and He calls us to a sobering responsibility here in this life. (Seven of eight NT references using the word sober are exhortations to the believer to be sober in spirit.) The work is in learning what His level of available resource is for us (unlimited in all things He desires for us) balanced against what our level of responsibility is in appropriating His resources. This work is not done to gain salvation, but follows for all of the rest of life from salvation or the moment of understanding and receiving His atonement.
Work out...something we do in the gym or on the field or in the basement to build strength.
Placing yourself in the rest of Jesus within, through faith in His work on the Cross is a deep work.
The working out of our salvation builds our strength in the spirit.