Paul was very optimistic and confident of what God can do in the lives of the Philippians, as an individual and the Church, in return, for their services to God and joining him as business partners of the gospel since the day they believed in Christ Jesus. He prayed and assured them that the 'good work' of salvation that God began in them will be perfected until Christ return. Paul was confidence to say this because he knew that whatever God begins he finishes it.
Paul is speaking to believers today that the good work of justification, sanctification, and glorification that God has began in us will be a continuous process in Him and in sharing the gospel with others. This is the confidence we have in Him, that God who has been sustaining us will definitely keep us until the very end.
Therefore, we do not need to be discouraged of whatever we do now that is not fruitful or not coming forth as we want it. Faithful is our God who will surely perfect whatever he begins. Be focused, press on, and be hopeful that all you do for the gospel will not be in vain. Amen