In the first five verses of his letter, Paul set out the Gospel in a nutshell, and there establishes the truths he would defend throughout the epistle. He began by calling himself an apostle, one of those sent not by the church but directly by Jesus Christ. As an apostle, Paul was a plenipotentiary representative of the Lord and had full authority over the fledgling church. In Galatians, Paul would defend that authority over against those who were sent by mere men.
Moreover, Paul claimed that "all the brothers with me" agreed with him in what he was saying. The heretics, who claimed to bring a message from the other apostles in Jerusalem, were lying. This theme—the united witness of the true church against heresy—also would be a major concern in the letter.
The heretics diminished the work of Jesus Christ by saying that human works were necessary for salvation as a supplement to Christ's work. By diminishing His work, they diminished His person. But Paul asserts in verses 1 and 3 the equality of Christ and the Father. Both Christ and the Father sent Paul (v. 1), and Paul puts them on the same level. Both Christ and the Father send grace and peace (v. 3), and only God can do that.
Paul told them that they have grace and peace. The heretics were telling them that they did not really have grace and peace unless they added some good works to the Gospel. The heretics, thus, denied that all grace and all peace come from God. They were "troubling" the church, stealing away the peace Christ had died to give.
Paul said that our salvation rests entirely upon the work of Jesus Christ. He gave Himself for our sins (v. 4). How can anyone presume to add to this? Against the heretics, Paul said that Jesus has rescued us from the present evil age. If we needed to be rescued, clearly we could not rescue ourselves. The Gospel, according to Paul and according to the Bible, means God does all the work of rescuing. It also means that whose who are rescued by God are rescued once and for all. Those who deny this are part of the evil age, from which we have been rescued.
Paul did not hesitate to expose heresy which observed the Gospel. Do you follow that example? Do not allow your legitimate desire for unity to squelch your understanding of the need to protect the purity of the Gospel. Be supportive of those who, like Paul, speak forcefully against any and all false teaching.