Like sunlight reaches the innermost cracks, crevices, and cubbyholes so does God reach the innermost crevices of our soul. Throughout Scripture God is referred to as Light. He is the one true Light. Just like it's hard to keep out sunlight, it's hard/impossible to keep out the penetrating rays of God's light. Sure we can try to keep Him out, but it's His very nature to filter straight through our protective barriers. God is the ultimate Light searching deep into our souls.
The beauty comes when we recognize this and ask Him to reveal what He has found in our soul. Psalm 139:23-24 says, "Search me, O God, and know my heart. Try me and know my anxious thoughts. See if there be any hurtful way in me, and lead me in the way everlasting." The beauty of this prayer is that it opens us up to what God finds when He searches us. When you pray this be careful though! It can get a bit uncomfortable. He will find your anxious thoughts and impure motives. He is undertaking some work on our soul. At that point He begins the healing process. Notice I said He begins it. The work is His part not ours. We have done our part by asking Him to "search" and "examine." In essence it's a matter of being cooperative with Him.
Psalm 139:23-24 is such a powerful prayer if you truly mean it. Will you pray it? Will you allow Him access to search your soul? It really is the only way to true and lasting peace.