Those who have been crucified with Christ through baptism now no longer live under the law but live by the power of the Spirit of Christ living in them.
For Paul the law belongs to the old order of existence. This conviction undergirds one of the most provocative and challenging proposals in Galatians. Earlier Paul says that justification is not by "doing the works of the law" but by "faith in Christ Jesus" (2:16). The law had a place only until the coming of the promised offspring of Abraham, Christ Jesus (3:19-21). In his Letter to the Romans, Paul expands this argument to say that the law has to do with sin, and since sin has only to do with living persons, those who are dead because of their being united with the death of Christ Jesus through baptism are no longer subject to sin and the law (see especially Romans 6:1-6).
In this passage that same connection is made, but less explicitly. Those who have been crucified with Christ are now dead to the law and live by virtue of Christ living in them (see Galatians 2:19-20). The new creation in Christ Jesus is of such profound significance that those who belong to Christ are no longer under the law but now live guided by the presence and power of the Spirit (5:18, 25).
It is clear that for Paul this assertion is not just a matter of wishful thinking about future possibility. It is the conviction of a present reality that shapes the lives of the Galatian Christians for freedom that is active in love of the neighbor (see 5:6, 14). In 5:18 and 5:25, although it is difficult to see this in English translation, the original Greek makes clear that both of the conditions ("If you are led by the Spirit," and "If we live by the Spirit") are constructed in such a way as to show that they suppose not a future possibility but an actual present reality (see the similar assertion in Romans 6:11-14). It remains for the Galatian Christians, as for countless generations to come, to explore and experience what it really means that in fact, led and guided by the Spirit, they are no longer under the law and so are empowered to produce the fruit of the Spirit (see Galatians 5:22-23).
16 Live by the Spirit, I say, and do not gratify the desires of the flesh. 17For what the flesh desires is opposed to the Spirit, and what the Spirit desires is opposed to the flesh; for these are opposed to each other, to prevent you from doing what you want. 18But if you are led by the Spirit, you are not subject to the law.
New Revised Standard Version Bible: Anglicized Edition, copyright © 1989, 1995 National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved worldwide. http://nrsvbibles.org