Romans 15:19 (NIV)
By the power of signs and miracles, through the power of the Spirit. So from Jerusalem all the way around to Illyricum, I have fully proclaimed the gospel of Christ.
Paul wrote much of the New Testament. So how do we know that it is the trustworthy and infallible word of God? It is by the authenticating power of signs and miracles given through the power of the Holy Spirit to Paul and the other apostles through whom we have the full proclamation of the gospel of Christ.
2 Corinthians 12:12 (NIV)
The things that mark an apostle--signs, wonders and miracles--were done among you with great perseverance.
Signs and wonders are what authenticate the apostle. So do we have any apostles today? You tell me, where are the things that mark one? Should we be concerned about that? This next verse says that the apostles and prophets were the foundation of the church. Does a building need more than one foundation?
Ephesians 2:19-22 (NIV)
Consequently, you are no longer foreigners and aliens, but fellow citizens with God's people and members of God's household, built on the foundation of the apostles and prophets, with Christ Jesus himself as the chief cornerstone. In him the whole building is joined together and rises to become a holy temple in the Lord. And in him you too are being built together to become a dwelling in which God lives by his Spirit.
Of course there can only be one foundation and that is Jesus Christ.
1 Corinthians 3:11 (NIV)
For no one can lay any foundation other than the one already laid, which is Jesus Christ.
The truth of God's word has already been authenticated by supernatural signs and miracles. There is no more need to further shore it up. It stands alone as God's truth. So how is today's gospel messenger authenticated? First by whether his message conforms to God's already revealed word. Secondly by another modern day miracle, transformed lives.