"But when the time had fully come, God sent his Son, born of a woman, born under law..."
Paul was well-versed in Torah and Greek philosophy. When he moved to Athens during his missionary journey, it seemed that he had a desire to show off his academic and intellectual abilities through debating with a group of Epicurean and Stoic philosophers concerning the gospel. Perhaps, he wanted to input his knowledge into evangelism in Athens. He tried to transfer the gospel within the understanding of Greek philosophy that the Athenians were familiar with. But he fell into a swamp, finding himself unable to deliver the gospel effectively.
The results in Athens were disappointing. A church was not established, and not many people were aware of the gospel in the first place, making it difficult for them to accept this new idea. As a result, Paul the great apostle became a just babbler and failed to present any evidence of the gospel (Acts 17:17-18).
After this failure, once he moved to Corinth from Athens, he made a solid decision not to come to Corinthians with eloquence or superior wisdom for the gospel (1 Cor. 2:1-2).
Parousia (παρουσία) in His Time!
Paul used the unique terminology of 'the basic principles' to refer to all religious teachings and ideas that do not lead to salvation. This concept is used primarily by Paul to refer to the Law of Moses on his epistles. It was not only ‘the basic principles’ that he had to abandon in front of the profound power and destructive power of the gospel.
He had to abandon Torah, his pride, as a vulgar elementary knowledge and refused to serve it again. Paul didn't simply regret it when he was under the law, but it was detestable and humiliating time for him. He speaks of the time he was under the law as a slave of the rudiments (Gal. 4:3). And he firmly states that he will neither return to the basic principle, nor be a slave to it again (Gal. 4:9). He declares that he has obtained the right to be a child of God in the gospel, not as a servant of the law (Gal. 4:5). The contrast of words he uses were very conflicting, for the terms, a servant and an heir, can never be homogenized.
He says that this dramatic transition happened only through the Spirit of the Son, and the Spirit of the Son allowed sinners to call God 'Abba, Father'.
Paul says that none of this happened haphazardly.
Gal 4:4 says ‘when the time had fully come, God sent his Son’. It means that Jesus’ first coming did not happen by accident. According to His time, God always provides His providence and fulfills His plan. Just as God sent Jesus Christ in His own time, Jesus Christ’s parousia (παρουσία) will also happen in His time.