Just reading the Responsorial Psalm today, gives us the feeling of awe. God is the One Who has been with us from Day 1 - "You ... created my being, knit me together in my mother's womb" (v 13). And more than the physical, "already You knew my soul" (v 14).
That's why the psalmist says, God 'searches', God 'knows', God 'discerns', God 'marks' - i.e. 'winnowed/sifted' our innermost emotions and moral sensitivity (vv 1-3) - wherever we are, whatever we do, "all my ways lie open to You" (v 3). But where and how can we ever hide from God?! (Ps 139:7-8).
It is comforting to know that God knows our sinful self and yet he is ready to forgive us. This can move us to express our gratefulness to Him - "I thank You for the wonder of my being, for the wonders of all Your creation" (v 14).
St Paul openly confessed, in his letter to the Galatians, that he was "merciless ... in persecuting the Church of God" (v 13); the "damage" he did (v 14); and then acknowledged that "while I was still in my mother's womb... God ... had specially chosen me ... called me through His grace and chose to reveal His Son in me, so that I might preach the Good News" (vv 15-16).
Knowing that God knows all that he had done must have been very liberating for St Paul. He didn't need to hide his sins. God's grace overtook his sins!
Lord, let us rejoice in Your deep and profound knowledge of us.