The following is an excerpt from The MacArthur New Testament Commentary on Ephesians 5.

Apart from the truth in verse 18, which is the heart of Paul’s message, the book of Ephesians would appear to be legalistic. Every exhortation he gives would have to be fulfilled through the power of the flesh. Believers would need to rely on their own resources and strength to follow the great road map of the Christian life that the apostle presents in chapters 4–6—and would, of course, find themselves completely deficient. Christians cannot walk in humility, unity, separation, light, love, and wisdom apart from the energizing of the Holy Spirit. To walk without the Spirit is to walk unwisely and foolishly (see vv. 15–17). We can “be imitators of God, as beloved children” (5:1) only as we are filled with the Spirit (cf. John 15:5).

In 5:18–21 Paul first presents the contrast of the way of the flesh with the way of the Spirit. The way of the flesh is characterized by the pagan religion out of which many of the Ephesian believers had come, a religion that centered around drunken, immoral orgies of supposed ecstasy, in which a person tried to progressively elevate himself into communion with the gods. It is the way of self, pride, immorality, greed, idolatry, confusion, deception, fantasy, falsehood, and even demonism. It is the way of darkness and foolishness (see 5:3–17).

In vv. 18b–21 the apostle gives the other side of the contrast—the godly walk of God’s children that expresses itself in the Spirit–controlled life and worship of beauty and holiness. He first gives the central command of the epistle (which is the focal point of the New Testament for believers) and follows it with an outline of the consequences of obedience to that command.

but be filled with the Spirit, (5:18b)

Although Paul was not present when the Holy Spirit manifested Himself so powerfully at Pentecost, he must have had that event in mind as he wrote be filled with the Spirit. Pentecost obviously occurred while he was still an unbeliever and before he began persecuting the church. But without Pentecost he and other unbelievers would have had no reason to persecute the church, because it would have been too weak and powerless to threaten Satan’s domain. It was there that the other apostles heard the heavenly “noise like a violent, rushing wind,” saw “tongues as of fire distributing themselves” and resting “each one of them,” and were “filled with the Holy Spirit and began to speak with other tongues, as the Spirit was giving them utterance” (Acts 2:2–4). It was also there that some of the crowd accused the apostles of being “full of sweet wine” (v. 13), probably expecting them to break out into the typical frenzied antics of mystical pagan worship. 

Though others (such as Moses, Ex. 31:3; 35:31) had been filled with the Spirit for special purposes, it was at Pentecost that all believers in the church were first filled with the Holy Spirit. Every promise that Jesus gave to His disciples on the last night He was with them was fulfilled in some sense by the coming the Holy Spirit on that day. In fact, it was the coming of the Holy Spirit that made real all the promises of Jesus Christ.

Jesus said, “And I will ask the Father, and He will give you another Helper, that He may be with you forever; that is the Spirit of truth, whom the world cannot receive, because it does not behold Him or know Him, but you know Him because He abides with you, and will be in you” (John 14:16–17). The Holy Spirit’s permanently indwelling all believers—rather than only being with some of them, as was true before Pentecost—is one of the great dispensational truths of the New Testament. In the new age, the church age, the Spirit of God would not be just be alongside His people but in them all (cf. 1 Cor. 3:16; 6:19).  It is this residence of the Holy Spirit in believers that makes possible the fulfillment of all Jesus' other promises to His people, and in Ephesians 1:13 He is called “the Holy Spirit of promise.” 

The Holy Spirit is our divine pledge and security that Jesus’ promises are fulfilled (2 Cor. 5:5). Among many other things, He guarantees and gives assurance that we will have a heavenly dwelling place in the Father’s house (John 14:2–3); that we will do greater works, not in kind but in extent, even than He did (14:12; cf. Matt. 28:18–20; Acts 1:8); that whatever we ask in His name he will do (John 14:13–14); that we will have Christ’s own peace (14:27); that the fullness of His joy will be in us (15:11). The Holy Spirit assures us that Jesus Christ and the Father are one (14:20); that we are indeed God’s children (Rom. 8:16); that he will intercede for us, making our prayers effective (Rom. 8:26); and that He will bear fruit in our lives (Gal. 5:22–23).

But the work of the Holy Spirit in us and on our behalf can be appropriated only as He fills us. Every Christian is indwelt by the Holy Spirit and has the potential of receiving the fulfillment of all Christ’s promises to those who belong to Him. But no Christian will have those promises fulfilled who is not under the full control of the Holy Spirit. We have just claim to all Christ’s promises the moment we believe in Him, but we cannot have their fulfillment until we allow His Spirit to fill us and control us. Unless we know what it is to be directed by the Holy Spirit, we will never know the bliss of the assurance of heaven, or the joy of effective work for the Lord, of having our prayers answered constantly, or of indulging in the fullness of God’s own love, joy, and peace within us.