What do you get when you take a bunch of sinful, selfish, passionate, suffering, and yet abundantly joyful people? The church! There is one thing that unifies us as the church: faith in Jesus Christ. Sometimes there is more than that, but there is never less than that. The Galatian church was at odds with one another (and with Paul) because of some errant theological convictions. We are vulnerable to the same problem.
There are plenty of outside agendas that put the simplicity of the gospel at risk. Cultural pressures, unbelief, rights and freedoms changing, persecution (overt or otherwise), and false teachers to name a few. The church is well aware of these most of the time, and is usually prepared to defend against the attacks of the enemy. We often forget, however, about the possibility of attack from within that causes the gospel to become watered down. This can happen when the simplicity with the gospel is encumbered with whatever passion any individual Christian might impose on the church at large. This could be something potentially amoral like diet choices, schooling options for your children or your political affiliations. This can also happen with morality based social justice activities like freeing people from the horrors sex trafficking or adopting and caring for orphans. None of these things are bad things, but none of these things are the main thing that the church is to be about.
Whenever good works are elevated to the level of gospel truth, the gospel is lost. Jesus is no less jealous for His own glory now in the life if the New Testament church than he has ever been throughout the ages. If we allow other activities or passions to push Jesus out of the center, we are propagating an alternate gospel, which is no gospel at all. Paul says that this anti-gospel emphasis is like leaven and goes throughout the whole dough. It impacts our willingness to love each other and to bear one another’s burdens. It infuses pride into our church which simply creates divisions and groups and sub-groups.
We need the book of Galatians to remind us of the simplicity of the gospel that has united us as the Church of Christ. What really matters when you distill it all down is that we are a group of people who have seen our sin and our inability to rescue ourselves and we have trusted in the Great Rescuer who gives it all freely to us as a gift of His grace.