Final answer:
Christ is the source of the apostolate because he sent out his disciples to propagate his teachings which laid the foundation for Christianity. His role as a divine intermediary is depicted in Christian art and doctrines, extending his message through the apostles to the world.
Step-by-step explanation:
Christ is considered the source of the apostolate because he was the one who sent out his disciples, termed as apostles from the Greek apostolos, meaning 'one sent, messenger, those sent by Christ to spread his teachings outside the Holy Land.' According to the Christian tradition and the gospels, after Jesus resurrected and before ascending into heaven, he re-emphasized his teachings and the offer of salvation, thereby laying the foundation for the spread of Christianity through his apostles.
The role of the pope in the church, the authority of which is believed to be derived from the Petrine Succession, and the early Christian emphasis on doctrine further anchor the significance of Christ's teachings as the core of the apostolate. In artworks such as the Christ Pantokrator and the Communion of the Apostles, and through historical interpretations such as those by early Christian writers like Justin Martyr, Christ is portrayed as the divine intermediary, extending his authority and message through the apostles.
Thus, the claim that Christ is the source of the apostolate is rooted in the belief that his life, resurrection, and establishment of his disciples as carriers of his message, mark the beginning of the Christian mission and mandate to disseminate his teachings throughout the world, which continues to be a central aspect of Christian ecclesiology.