The verse you quoted (Deuteronomy 29:29) is an excellent verse which shows that God keeps secret things and God also REVEALS secret things. This is His program for the end of the world. The Lord told the prophet Daniel to seal up the word until the time of the end--and then knowledge would be increased:
Daniel 12:4 But thou, O Daniel, shut up the words, and seal the book, even to the time of the end: many shall run to and fro, and knowledge shall be increased.
This verse in Daniel 12:4 basically shows us exactly what Deuteronomy 29:29 was referring to. Secret things (shut up the words) belong to God and according to His program of times and seasons He is able to reveal them (knowledge shall increase) as He so wills to do so.
You see the problem you and many in the churches are having is that you think you control God's Word. Church theologians believe that God cannot open up His Word and reveal any further truth than what they've already discovered in centuries past. This idea shows the arrogance of the natural minded theologians that have operated within the churches and congregations. They approach the Bible as a lifeless (dead) book that has revealed all of its secrets long ago. But the truth is that God's Spirit is very active in our day as He is continuing to guide His elect people into more and more truth. And many of these truths have never before been known or understood.