As Christ's Spirit directed the prophets to write their prophecies, they wanted to know "who" and "when" the Spirit was referring to. Upon suffering and being glorified, who would bring this salvation? When will it take place? According to this statement, the Old Testament prophets did not write their own ideas. This verse also reinforces another crucial idea: The ancient prophecies promised a Savior who would suffer and then be glorified.
Despite their joy over the coming Messiah sitting on David's throne and saving the people from their sins, the prophets diligently inquired as to what time and circumstances the Holy Spirit was indicating when He told them in advance about the terrible suffering to come. It is all contained in the Word of God as God has spoken to us in these last days through His Son.
It is truly sad that so many are looking for additional illumination outside of the Bible to discover His plans and purposes for them. It is imperative that we study to prove ourselves acceptable to God and not seek wisdom from any authority other than that of God's inerrant Word.

Dear Heavenly Father, thank You for providing everything I need for life and godliness in Your Word. In times past, You kept secret things from Your church, but Your Son, our Savior Jesus Christ, has revealed them to us through Your Word. In the Holy Scriptures, You have revealed all we need so that we can make wise choices for salvation. We are grateful that spiritual illumination or extra-biblical revelation is not required elsewhere. 
In Jesus's name I pray. Amen.