21 For our heart [leb] shall rejoice [samach] in him, because we have trusted [batach] in his holy [qodesh] name [shem]. KJV-Interlinear
21 For our heart is glad in him, because we trust in his holy name. ESV
The word for heart, ‘leb’ is symbolism for the thinking process of the soul.
In the physical body, the heart is the pump that circulates the blood throughout the body. It describes the thought process the thinking process, and all of the functionality of the soul.
And when your thoughts are such that the spiritual truths that you have learned, predominate and/or occupy the lions share of your thinking process, then there is only one possible result. And that result is inner happiness.
And happiness, the only legitimate happiness and legitimate happiness comes from God. And we come into possession of those thoughts and that happiness through our learning of Bible doctrine.
You have only two possible sources for your thoughts, Bible doctrine, or everything else.
The things you focus on in your daily life, day after day after day becomes the basis for the pattern of your life.
The world offers nothing holy, only things that are temporary.
Holy means separate from, sanctuary, consecrated, hallowed. And all of this comes from and is, God.
God is not a part of this world, not a part of the evils that the world represents.
Likewise Bible doctrine is not a part of this world or the evils that are in this world.
When you bring Bible doctrine into your soul through your daily learning, then your soul becomes, symbolically, separated from the content of the world, and a sanctuary or protected place into which the world cannot enter.
Excluding Bible doctrine from your life, means that you exclude legitimate happiness from your life, and when there is no happiness, no legitimate happiness, then there is only misery.