John 16:12-15
12 "I have much more to tell you, but you cannot bear it now.
13 But when he comes, the Spirit of truth, he will guide you to all truth. He will not speak on his own, but he will speak what he hears, and will declare to you the things that are coming.
14 He will glorify me, because he will take from what is mine and declare it to you.
15 Everything that the Father has is mine; for this reason I told you that he will take from what is mine and declare it to you.
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Even after three years, the Twelve were not able to bear the things that Christ needed to teach them.
They had three years of walking with Christ, talking to him day in and day out, with a front row seat to his miracles, and they're still not ready. They needed the Holy Spirit to guide them in all truth.
So it is with us. We NEED the Holy Spirit if we want to understand the height and breadth and width and depth of what God desires to give to us. I used to think I could just bear down and study the Scriptures and "get it." That's what has worked for me my whole life -- I was a straight A student in high school, college, and grad school, and that's always been the path to success.
The desire to learn and grow is good, and I think it is absolutely necessary to grow spiritually. But rather than rely on myself, I needed to take that desire and hand it to God, and ask Him to send His Spirit to teach me.
Put another way, I had to learn that men don't grow spirit -- spirit grows spirit. Coming to that understanding has been a huge milestone in my own spiritual development.