Final answer:
The crowd observed the apostles speaking in different languages on Pentecost due to the inspiration of the Holy Spirit, which allowed them to communicate Jesus's teachings to people of varied linguistic backgrounds.
Step-by-step explanation:
The crowd noticed that on Pentecost, the apostles were speaking in different languages. This miraculous event is described in the Acts of the Apostles in the New Testament, where the Holy Spirit descends upon the apostles in the form of tongues of fire, and inspires them to preach about Jesus Christ in various languages so that people of different tongues and tribes could understand the message. This event fulfilled Jesus Christ's promise to send a helper, the Holy Spirit, to guide the apostles after His ascension into heaven. Moreover, this phenomenon allowed the apostles to spread Jesus's teachings beyond the Holy Land, in accordance with their role as messengers or apostles from Greek apostolos, meaning "one sent".