The word “THEN”, opening this verse serves as a conclusion to what came immediately before in the previous 4 verses. IF – we seek out God’s Word with all due diligence – THEN we will understand THE FEAR OF THE LORD. If the fear of the Lord is the very beginning of knowledge (Prov. 1:7), then this is how we come to it. The pursuit laid out for us in the first part of this chapter is the very means to gain this fear. Combine the two and you have this simple reality – I cannot know God rightly, unless I pursue the knowledge of Him the way He has appointed. You cannot know anyone, let alone God, apart from taking the steps necessary to understand their heart and mind and values and goals and desires. And we cannot know these things about God apart from the diligent searching out of His Word by the aid of the Holy Spirit.
The amount of the Bible you are willing to leave un-mined – is directly proportional to the amount of God you are unwilling to know, and willing to do without. Note again this verse begins with “THEN” – THEN you will understand, and not until.