“But the ones (seed) that fell on the good ground are those who, having heard the word with a noble and good heart, keep it and bear fruit with patience.”   Luke 8:15

“For with the heart one believes and is justified, and with the mouth one confesses and is saved.”   Romans 10:10

The heart!
“I the LORD search the heart and test the mind…”   Jeremiah 17:10
Nothing is hidden from God!

“And you Solomon my son, know the God of your father and serve him with a whole heart and with a willing mind, for the LORD searches all hearts and understands every plan and thought…”   1Chronicles 28:9

“Can a man hide himself in secret places so that I cannot see him? declares the LORD.   Do I not fill heaven and earth? declares the LORD.”    
Jeremiah 23:24

“Hell and Destruction are before the LORD,
So how much more the hearts of the son of men.”   Proverbs 15:11

Jesus knew the hearts of men as we see in the Gospels:
(I’ll leave it to you to finish each verse.)
“But Jesus knew their thoughts…” Matthew 12:25
“But Jesus perceived their wickedness…” Matthew 22:18
“And immediately Jesus perceiving in His spirit that they thus questioned within themselves…”   Mark 2:8
“But He knew their thoughts…”   Luke 6:8

The Lord knows the believing hearts and He also knows who professes to be His, but do not believe in their hearts nor keep His word.   Scripture has much to say of the heart and He admonishes His own to keep a guard on this place that only God can truly and fully know.  
For He says in Proverbs 4:23,
“Keep your heart with all diligence,
For out of it springs the issues of life.”

Proverbs 23:7, “For as he thinks in his heart, so is he…”    

Jesus tells us that evil comes from the heart, Matthew 15:18-9
“But the things that proceed out 0n the mouth come from the heart, and these defile a man.   For out of the heart come evil thoughts, murders, adulteries, immorality, theft, false witness and slander.”

The heart controls speech as He tells us in Luke 6:45,
“A good man out of the good treasure of his heart brings forth good; and an evil man out of the evil treasure of his heart brings forth evil.   For out of the abundance of the heart the mouth speaks.”

What then of the heart of the believer?   We are to given our heart wholly to God--
In love:
“…You shall love the Lord your God with all your heart and with all your soul and with all your mind.
This is the great and first commandment.
And a second is like it: you shall love your neighbor as yourself.”  
Matthew 22:37-39

In obedience:
“Blessed are those who keep His testimonies,
Who seek Him with a whole heart!
They also do no iniquity;
They walk in His ways.”   Psalm 119:2-3

In trust:
“Trust in the LORD with all your heart,
And lean not on your own understanding.”   Proverb 3:5

In prayer:
“And you shall seek Me, and you shall find Me, when you shall search for Me with all your heart.” Jeremiah 29:11

So with believing, responding and obedient hearts let us humbly come before Holy God giving Him praise, honor and glory.   Thank You Father for loving us!   Thank You Jesus for redeeming us!   Thank You Holy Spirit for your mighty work in us moment by moment!   What an awesome thing it is to belong to God!     Amen