Now I say, [That] the heir, as long as he is a child, differeth nothing from a servant, though he be lord of all;
Note 1 at Ga 4:1: Here, Paul gave a human illustration to show the state of spiritual immaturity that people experience by being under the Law. Paul stated that being under the Law is no better than being a slave, whereas faith in Christ brings people into a position of full-grown sons (Ga 3:26).
It is sad to say, but most Christians have never graduated into the son-ship that Paul was describing here. They are still serving God with an Old-Testament-Law mentality; that is not pleasing to God (Heb 11:6) and is oppressive to them (see note 4 at Ro 3:19). The truth is that son-ship is theirs. It's been provided for them freely through Christ, but it doesn't happen automatically. They have to renew their minds (see note 9 at Ro 12:2) to take advantage of this blessed relationship.