To be filled with the Holy Spirit means the word of Christ dwells in us richly. It means that we are abiding in Christ and in His word. We cannot separate Christ from the Holy Spirit as we cannot separate the Holy Spirit from Scripture.
God’s commandments are for our good and His glory. God, through Paul, issues a command to His beloved children to be filled with the Spirit. We must understand this is not an option or an add-on for the Christian. We must be filled with the Holy Spirit. We cannot live the Christian life apart from it.
We were darkness but now we are light in the Lord (5:8). The text does not say we were in darkness but rather we were darkness. Our lives were characterized by sin and ignorance of God’s truth. But that is not who we are now. Now we are light. We are holiness and we have been given knowledge of God’s truth.
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.