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Many believers think that the Lord is mysterious.

O the depth of the riches both of the wisdom and knowledge of the Lord! how unsearchable are his judgments, and his ways past finding out! -

Understanding the second part of this verse requires understanding of the first part of the verse.
→ His judgments are unsearchable in the sense that they are never exhausted.
→ His ways are past finding out in the sense that there is always more of Him to explore.

Our Lord has not hidden anything from us. We are part of Him in the New Birth by the Word and the Holy Spirit.

So we see that there is always more to explore in our Lord but He is not mysterious.
But grow in grace, and in the knowledge of our Lord and Saviour Jesus Christ.     To him be glory both now and for ever.     Amen. -
What is the source of knowledge of Jesus (who is the Truth)?
1) the Word Of The Lord which is Truth
2) the Holy Spirit who is the Spirit Of Truth.
When we spend time meditating on the Word Of The Lord the Holy Spirit reveals more Truth to us which enables us to appropriate more of the Lord's Grace Through Faith.

The eyes of your understanding being enlightened; that you may know what is the hope of his calling, and what the riches of the glory of his inheritance in the saints, -
When your understanding is perfect you can visualize what you know.
→ You begin to see into the Spirit Realm the Truth of the Lord's Word by revelation of the Holy Spirit.
→ You begin to fathom what it means to be In Christ, to be a joint-heir with Christ.
→ You begin to be an effective witness for Christ because of that revelation.

As we hunger and thirst for the Lord we are filled.
Blessed are they which do hunger and thirst after righteousness: for they shall be filled. -
This is an if-and-only-if statement by Jesus.     He is saying that only those who hunger and thirst after righteousness will be filled.     Those who do not hunger and thirst can not and will not be filled.
Draw near to the Lord and He will draw near to you.
I love them that love me; and those that seek me early shall find me.     -

Furthermore, our Creator appeared in human flesh. His name is Jesus. And then when He left He sent His Spirit to comfort, teach, help and empower us.

So what is mysterious about Him? Nothing, unless you believe there is another God who never shows Himself and never comes around but somehow simply controls all things and sends representatives like Jesus and the Holy Spirit in His place.