Galatians 5:13

For brethren, ye have been called unto liberty
He calls them "brethren", to testify his affection to them, and to put them in mind of their relation to one another, which required mutual love, a thing he is about to press them to; he asserts that they were "called" not merely externally, but internally, by the effectual grace of God, out of bondage to sin, Satan, and the law, unto the liberty of the Gospel and of the grace of God; that liberty wherewith Christ had made them free, ( Galatians 5:1 ) this he said in a judgment of charity, hoping well of them:

only use not liberty for an occasion to the flesh;
corrupt nature, which in unregenerate men takes encouragement from, and makes an ill use of the best of things, as the mercy and patience of God; and not only takes an occasion by the law, forbidding sin to work and stir up all manner of concupiscence; but also by the Gospel, and the doctrines of it, such as eternal election, free justification which though the source and fountain, the barrier and security, of all true and real holiness, are improved and abused by wicked minds, under the influence and instigation of Satan, to vile purposes; and though regenerate persons are not in the flesh, and do not live after it, yet that is in them, and there is a proneness in them to sin; and Satan is watching all opportunities and advantages against them; so that there is need for such a caution as this, that they do not misuse their Christian liberty by indulging the flesh and the lusts of it, which is apt to take an occasion to cherish its lusts, and especially when given: Christ's free men should not do so, for that is to disgrace the doctrine of Christian liberty, to enthral themselves in, bondage instead of using their liberty aright, and to give the enemy occasion to blaspheme: the doctrine of Christian liberty may bc abused, or used as an occasion to the flesh, and to fulfil the lusts of it; when under a pretence thereof men think themselves exempt from obedience to the civil magistrate, which is using this liberty as a cloak of maliciousness; or that they are free from obedience to the law of God, as a rule of walk and conversation; or from subjection to the ordinances of the Gospel; or when they use the creatures God has given them the free use of without thankfulness, and in an immoderate manner; and when they make things indifferent which are not, or use indifferent things to the prejudice of others; and their liberty becomes a stumblingblock to weak Christians, which the apostle seems greatly to regard here; since he adds,

but by love serve one another:
the Vulgate Latin version reads, "by the love of the Spirit": and so some copies; Gospel liberty and the service of the saints are not at all inconsistent; as it becomes them to love one another, as the new command of Christ, their profession of religion, and their relation to each other, require, so they should show their love by their service; as by praying one with and for another, by bearing each other's burdens, sympathizing with and communicating to each other in things temporal and spiritual; in forbearing with and forgiving one another; by admonishing each other when there is occasion for it, in a meek, tender, and brotherly way; by instructing and building up one another on their most holy faith, and by stirring up one another to all the duties of religion, private and public.