What is the Protestant interpretation of Philippians 3:12?

Protestantism generally teaches that salvation is by faith alone, through grace. Paul seem to make a case for this in Philippians 3:9: > and be found in him, not having a righteousness of my own that comes from the law, but that which comes through faith in Christ, the righteousness from God that depends on faith (ESV) but later on verse 12, he says: > Not that I have already obtained, or am already made perfect: but I press on, if so be that I may lay hold on that for which also I was laid hold on by Christ Jesus. Does that not seem to indicate that we cannot be sure of our salvation, and that works are also implied?