I find it interesting that there seems to be parallels between Genesis 21 and the nascent New Testament church. I see the weaning of Isaac (Genesis 21:8) foreshadowing the coming of age of the Grecian believers in Acts 6 & 7. There had been a rivalry between the Jewish authorities and the Apostolic Church, whereby the Jewish authorities exalted their position over that of the disciples. Through prayer the nascent Church (Sarah) was able to discipline the Jewish authorities (Hagar – cp. Galatians 4:25), first by causing the death of Jewish spies (false brethren) who had infiltrated the Church (Acts 5:1-13), and secondly by making the authorities fearful of reprisals from the people (Acts 5:26, 38-39).
Due to the persecution of the disciples by the Jewish authorities (Acts 8-9), the Church prayed and Caligula threatened Jerusalem with banishment and dispersion. Indeed, war with Rome was eminent in 39 AD, almost 30 years before it actually occurred in 66 AD (cp. Luke 21:20). Caligula had demanded that his image be placed in the Temple at Jerusalem, which could never have occurred without the Jews revolting against Rome. Consequently, the Jewish authorities, preoccupied with the Roman threat, stopped their persecution and repented before God. As a result, Caligula was assassinated, and the new emperor, Claudius, appointed Herod Agrippa king over Judea.
The rivalry between Jewish authorities and the Church arose once more a few years later when James, the brother of John (Acts 12), was killed by Herod. The Church prayed and the Herod Agrippa, King of the Jews, was struck by God and killed. This resulted in the Jewish nation once again becoming a province of Rome. Rivalry broke out 10 to 12 years later with the imprisonment of Paul over a misunderstanding that developed surrounding his worship in the Temple, but the Jewish authorities, seeking to take advantage of the event, sought to falsify the charges and execute him. The Church prayed, and Jonathan, the high priest who had become obsessed with Paul’s death, was himself murdered in the Temple by Jewish zealot who had been bribed by Felix, governor of Judea.
Finally, the rivalry between Jewish authorities (Hagar) and the Church (Sarah) arose once more when the high priest, Ananias, killed James, the Lord’s brother. The Church protested in vain to the new Roman governor, but they also prayed, and events were set in motion that culminated in the destruction of the Temple and the dispersion / banishment of the Jews (cp. Genesis 21:10). The account in Genesis 21 is brief, but it is not too difficult to understand that the rivalry between Sarah and Hagar was longer and more drawn out than what one might think after only a cursory read.
In Genesis 21:9 Ishmael seems to express the sentiment of the ongoing rivalry between his mother, Hagar, and Sarah. No doubt Hagar trusted that their position (i.e. Hagar’s and Ishmael’s) with Abraham was secure. We are told that Ishmael was mocking Isaac over his being weaned. Most translations go along with mocking, but the same word is used for both Abraham and Sarah laughing. It is the context that shows Ishmael was mocking or making sport of the event. He was not delighted with the attention Isaac was given, and more than likely this expressed Hagar’s own sentiment.
Understanding that laughter in this case represents not only displeasure in what was taking place but also over confidence in one’s position, a parallel can be seen in Acts 6-9. There the Grecian believers were becoming of age (weaned of the motherly Apostolic Church) and almost immediately were persecuted with Saul playing the part of Ishmael (cp. Acts 8:1, 3). Over against this persecution / mocking or laughter, we can understand Jesus’ promise in Luke 6:21-22, which shows that those who mourn now (over the present condition of the Kingdom of God) will laugh, but those who laugh now (i.e. are content in their position that opposes the Kingdom of God) will mourn later (cp. Acts 8-9 & Genesis 21:8-9).
 Josephus, Antiquities 18.8.2
 Josephus, Antiquities 18.8.3
 Josephus, Antiquities 18.8.9; 19.5.1
 Josephus, Antiquities 20.8.5
 Josephus, Antiquities 20.9.1-2