Jesus promised to teach his apostles and he promised they would remember what he taught them (John 14:26; 16:13). "Do not worry about what to say or how to say it," he told them. "At that time you will be given what to say, for it will not be you speaking, but the Spirit of your Father speaking through you" (Matthew 10:19-20; cf. Mark 13:11; Luke 12:11-12). During stressful situations, Jesus promised, God the Holy Spirit would guide their words.
The promise was also meant for those apostles who participated in the writing of the New Testament. During the Last Supper, Jesus told them, "But the Counselor, the Holy Spirit, whom the Father will send in my name, will teach you all things and will remind you of everythingI have said to you" (John 14:26). Later he said, "But when he, the Spirit of truth, comes, he will guide you into all truth" (John 16:13). Jesus was reassuring his apostles that their written accounts would be God-breathed - i.e., inspired. Thus, according to the text, the apostles were promised the presence of God when they spoke or wrote God's message. Whatever they said or wrote, the apostles were inspired. This promise was made only to the apostles, not to all Christians (Acts 2:43; 5:12). Only the apostles, and those upon whom they laid hands, performed miracles. These confirmed and proved that their message was from God.
These apostles, along with their prophetic associates (e.g., Luke, Jude, and James), became the writers of the New Testament. There were also prophets, such as Agabus, who spoke their prophecies (Acts 11:28). The apostle John referred to himself as a prophet (Revelation 22:9).
Reference: The Everything History of the Bible Book, Jeffery Donley, Ph.D., Adams Media.