What parts of the Bible do we need a messenger to understand?
The "Iglesia Ni Cristo" teaches that the Bible cannot be understood without the help of a messenger, but they don't claim that every aspect of the Bible needs this special attention. Which parts of the Bible can one understand on their own and which parts need a minister?
When we examine the INC's doctrines and the existence of other churches that share some of these doctrines, we find that the logical conclusion of the INC's teachings is that nearly all of the Bible can be understood without the help of a messenger.
The INC's teachings are only unique on prophecy and some obscure verses.
Concerning passages, the INC's understanding is most notably unique on some prophetic passages. The INC interprets many of them to refer to Felix Y Manalo and the Philippines. Beyond these prophetic passages, which they consider to regard Felix Y Manalo, their understandings of passages are only unique in a way that does not affect doctrine. For example, the INC believes Ephesians 2:14-15 to speak of the union between Jesus and the church, although practically every other church considers it to speak of the union between Jews and Gentiles. Doctrinally, this has no significance since other churches also agree that the church is Christ's body and they are united.
The INC's teachings are only unique concerning the messenger.
Obviously, there is no other church that believes Felix Y Manalo to be God's messenger in these last days. Beyond this one point, nothing that the INC teaches is unique. There are others who read scripture and conclude that Jesus is not God. There are others who read scripture and conclude that Jesus did not have a pre-existence. There are others who read scripture and conclude that faith is not sufficient for salvation. There are others who read the Bible and conclude it is not permissible to eat blood.[1]
The messenger is only needed to help one understand the doctrine of the messenger.
Since the only truly unique teaching of the INC is that Felix Y Manalo is God's messenger in these last days, we must conclude that this is the only thing a messenger is required to reveal. It is obvious that all other doctrines taught by the INC can be concluded by one's own attempt to understand scripture as evidenced by the fact that others do arrive at the same conclusions and have done so before the INC was founded.
Someone can understand the gospel without the help of a messenger.
Since all doctrines except that of the messenger doctrine may be concluded without the help of a messenger, the saving gospel of Jesus Christ can be understood without the help of the messenger. Let us therefore read the scripture diligently and understand it even when there is no messenger to help us.
2 Corinthians 5:21 (ESV) - 21 For our sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God.
Conclusion
The INC's doctrine of a messenger is ill-defined, meaningless, and foolish. It is ill-defined in that they have not made any claims about which aspects of the Bible require a messenger to explain. It is meaningless in that the logical conclusion of the doctrine is that nearly all of the Bible can be understood without the help of a messenger. It is foolish in that we should not trust men over God, but be diligent to search the scriptures ourselves.
Acts 17:10-11 (ESV) - 10 The brothers immediately sent Paul and Silas away by night to Berea, and when they arrived they went into the Jewish synagogue. 11 Now these Jews were more noble than those in Thessalonica; they received the word with all eagerness, examining the Scriptures daily to see if these things were so.
[1] This is not to say that these are correct teachings, just that some others have arrived at the same conclusion.