I confess that it sounds strange that we can be complete because of times of trials and sufferings in our lives. There are times I feel like the picture that I'm in a dry place and have no clue as to what to do or where to go next. I am at my end, tired of suffering.
Paul is confident in what God is doing with the Philippian church, while he is in prison. He then talks about how through his imprisonment the gospel is spreading.
"These things I have spoken to you, so that in Me you may have peace. In the world you have tribulation, but take courage; I have overcome the world." John 16:33
So what is the work that Paul is talking about? On the day that we accepted Christ into our lives, the Holy Spirit started giving us a makeover. He is busy at work in our lives transforming us into the likeness of Christ.
That doesn't mean we will look like Him on the out side. It is an inward transformation, changing our hearts and minds into that of Christ. This is the work that God is perfecting in us daily.
We can trust that in our sufferings, God is using them to perfect us. It may not feel very comfortable at that time. In the end of that time you will be more like Christ than when you started.