First, it is important to pause and notice that Paul wants us to fix our gaze on “him”, which in the context means the Father. What Paul wrote in 3:14-19 and what he writes in 20-21 is all about God. When we get to the next lines of the verse, it is not about what God might do through us but about God’s work.
How many times have you received good news for your prayers, only to turn quickly from praising God to asking yourself when things will go wrong? We have all done this. It is possible for us to believe neither what God says nor the way He answers our prayers.