"Bear[ing] fruits worthy of repentance" implies a process. Just as a tree does not produce fruit overnight, a Christian does not fully repent overnight. It is a lifelong process of making changes, and over time we will produce the fruit of the Spirit more consistently than the works of the flesh.
Martin G. Collins
Basic Doctrines: Repentance
Verse 18 appears as a summary statement in light of all that Paul wrote previous to this. It needs a bit of defining. According to what the apostle wrote earlier, to be "under the law" includes three areas:
1. Most obviously, it means to be under the law's penalty because we have sinned. Jesus died so that we can be freed from that penalty.
2. It means to be striving to achieve justification through lawkeeping, which is what the main body of this epistle covers.
3. The third meaning is also covered but less thoroughly: that a person is trying to earn God's election and salvation by becoming a member of the Old Covenant. Chapter 5 covers that to a very small extent.
Paul's statement, then, must be seen in context of all that has been written before. Notice what Kenneth Wuest writes in Word Studies in the Greek New Testament, Volume 1, page 156. This is a typical Protestant statement regarding verse 18.
The exhortation is therefore, to be led by the Spirit. The assurance is given those who do so, that they will not be living their lives on the principle of legalism. The Spirit and the law are here contrasted, and are shown to be methods of living a Christian life that are diametrically opposed to one another. The law is not only no safeguard against the flesh, but rather provokes it to more sin. Therefore, the believer who would renounce the flesh, must renounce the law also. Thus, the flesh and the law are closely allied, whereas the flesh and the Spirit are diametrically opposed to one another. (Author's emphasis.)
To understand this truthfully, all he needs to do is reread what Paul wrote. What the apostle contrasts is Spirit with flesh, and Spirit with those under the law—not the law per se. But this commentator made no attempt to define what Paul means by "under the law," as Paul himself uses it in the epistle. Also, there was no attempt to define what the author of the commentary means by "legalism."
We have already seen what Paul means by "under the law." To these people, legalism is "the belief that one is obligated to obey the law." The key word in that definition is "obligated." They hate it (Romans 8:7), and therefore lawkeeping is seen as a burden, a yoke of bondage, despite the undeniable fact that God (through James) says it is a "law of liberty" (James 1:25; 2:12).
John W. Ritenbaugh
The Covenants, Grace, and Law (Part 28)
This verse is especially helpful in understanding the fruit of the Spirit because it directly precedes Paul's naming them: "But if you are led by the Spirit, you are not under the law." Being led by the Spirit (Romans 8:14) is a necessary precursor to producing the fruit of the Spirit in us.
John W. Ritenbaugh
The Fruit of the Spirit
Other Forerunner Commentary entries containing Galatians 5:18: