We have had the entire New Testament as the Word of God for almost 2000 years. Why do we need prophecy in addition? In the Old Covenant, God made the covenant with His people at Mount Sinai. However, God considered it necessary to send prophets to individuals, to rulers and to the whole nations after the covenant was made. All prophets in the Old Covenant, of whom we have a book in the Bible, worked after the covenant was made at Mount Sinai, the last of them, Zechariah and Malachi nearly 1000 years later. The goal of prophecy is therefore not the authentication of the Covenant. Rather, God speaks into concrete situations to warn of dangerous behavior and of His impending judgment, but also to encourage by a glimpse of His greatness and the future He wants to give. The same applies to the prophecies handed down to us in the New Testament.
The church of Jesus Christ is built on the foundation of the apostles and prophets (Eph 2:20). From the context of this passage it is very clear that Paul is not talking about the prophets of the Old Covenant here, but about living persons who do their ministry next to the apostles.
Originally, it was not God’s intention to create the Gospel as a book, because otherwise probably all the apostles would have contributed to the creation of the New Testament. However, among the authors of the New Testament, besides the apostle Paul, of the 12 disciples of Jesus, only Matthew, John and Peter are found; of the 9 others, except for a few short quotations in the Gospel of John, we do not have a single line in the New Testament. The New Testament was an emergency measure of the early church after the death of the apostles, in which all still tangible writings of apostles and their disciples were collected. Thus, the writings of the New Testament contain the proclamation of the apostles of Jesus Christ and are thus the standard for the doctrine and practice of the church and thus also for all prophetic words.
But also today God wants to speak into concrete situations in order to warn of dangerous behavior and of His impending judgment, but also to encourage by looking at His greatness and the future He wants to give. Christianity is not a book religion like Islam, in which God sends a book through a prophet, in which then everything is written that one has to follow, but a relationship with the living God, who wants to communicate with us today and so gladly wants to help us concretely.
If a husband had a wonderful correspondence with his wife before the wedding and they had well documented their joint decision to get married: Would the wife be happy about this if her husband reads to her only what they wrote to each other before the marriage and during the wedding, and and doesn’t say anything else to her? Would the husband be happy if his wife read to him only what she once wrote to him earlier, but had nothing else to say to him?
If prophecy is so important to God that He makes Paul to strongly recommend that we seek it diligently, then who are we to think we don’t need it?
If we forbid the Holy Spirit to speak directly to us or no longer allow Him to expose sins to us and others (see 1.Cor. 14, 22 – 25), the Holy Spirit may withdraw and no longer be willing to work in what we do and say. The result would be a church in which everything still seems to be in order and much is still going on, but which is ultimately dead and has no power to reach people’s hearts and in which, therefore, change by God no longer happens.