The Old Testament is the bottom line "foundation" of the New Testament. Look who's in it....
Elijah, Moses, Abraham, Jacob, David, Ruth, Rahab, Job, Deborah, Gideon, Boaz, Jabez, Hannah, and so many others. And look at the book of Isaiah. God is pouring out His heart all in that one. He tells widows and those rejected by their husbands (the outcasts) the HE, the living God, the God of Israel, is their Husband (in chapter 54).
It's like spiritual vitamins to delve into the Old Testament and gain a solid foundation in it. It's history. God's own history. It has God's promises, that begin right in the book of Genesis. And look at THIS promise. It's simultaneously DELIGHTFUL and eye-opening:
"Call unto me, and I will answer thee, and show thee great and mighty things, which thou knowest not." ~Jeremiah 33:3
This means that when we delve into "dig" into His Word, "calling unto" Him, "seeking Him" so to speak, His precious Holy Spirit WILL "teach us", as it is written, and "show us" (open our eyes) to what we didn't notice or "see" before (will SHOW us things we knew not). It's as if those words will be "illuminated".
In addition, in the Old Testament, where would you find God's own description of Job, and how it ties right into a verse in Proverbs like a puzzle piece fitting together to the book of Job, and is the "key" to understanding the entire book of Job?
The Old Testament is like that "hidden treasure" that is spoken of and is a prerequisite to understanding the New Testament.