Freedom in Christ...

For you were called to freedom, brothers. Only do not use your freedom as an opportunity for the flesh, but through love serve one another.  For the whole law is fulfilled in one word: "You shall love your neighbor as yourself." But if you bite and devour one another, watch out that you are not consumed by one another.—Galatians 5:13-15

The word “opportunity” used in verse 13 according to John MacArthur, was often used to refer to a central base of military operations.  The freedom Christians have is not to be used as a base from which they can sin freely and without consequence.  As born again believers, we ARE called to freedom...our freedom, however, is in Christ.

For the law of the Spirit of life in Christ Jesus has made us free from the law of sin and death. —Romans 8:2

Our freedom in Christ is to be used for the benefit of others and perhaps by God’s grace, win them to Christ.

For though I am free from all, I have made myself a servant to all, that I might win more of them. (1 Corinthians 9:19, ESV)

The key to this passage from my perspective is the word love used in conjunction with serving in verse 14.  The OT and the NT agree...love fulfills the law.  Love is the ruling principle of both.

"Teacher, which is the great commandment in the law?" Jesus said to him, "'You shall love the LORD your God with all your heart, with all your soul, and with all your mind.  "This is the first and great commandment. "And the second is like it: 'You shall love your neighbor as yourself.'  "On these two commandments hang all the Law and the Prophets."—Matthew 22:36-40

When legalism reigns, love is lacking.  The outward requirements of the law become more important than the inward. Legalism misses the heart.  It overlooks and dismisses the people Christ died to save.  The best imagery for me of what happens among believers in the spiritual realm when love is lacking is aptly stated in verse 15...we bite, devour and are consumed by one another. Pretty graphic picture and a stark warning.