First, I imagine God the Father, the first person of the Trinity in Heaven saying, “No sin is allowed in Heaven. This is my dwelling place. No original sin, no mortal sin, not even an ittsy bittsy teensy weensy venial sin is allowed here. God and sin do not mix.”
Second, I imagine Jesus Christ, God, the second person of the Trinity saying, "My mother, the Arc of the New Covenant, and my human body, will be my dwelling places. No sin can be in either place. No original sin, no mortal sin, and not even an ittsy bittsy teensy weensy venial sin is allowed. God and sin don’t mix.
Thirdly, I imagine the Holy Spirit, God, the third person of the Trinity saying, “Christian, you will be my dwelling place. You will be my temple. No original sin is allowed there. No mortal sin is allowed there. Not even an ittsy bittsy teensy weensy venial sin is…oh, wait…never mind. I can tolerate a venial sin in my dwelling place.” Christian responds, “Huh? God and sin do mix?!”
How does God, the third person of the Trinity, accomplish this? Even though mortal and venial sin are “different,” they are both still imperfect. Now, I can understand why the Holy Spirit might want to dwell in a newly baptized baby, or a Christian fresh out of the Sacrament of Reconciliation, and maybe even someone who just recieved the Holy Eucharist. But why would He “stick around” and continue to dwell in us when we’re not in a state of absolute perfection and impecability? How does He “mix” with imperfection in His temple? To me it seems like light and darkness trying to occupy the same “place.”
p.s. If possible, I would like explanations in layman’s terms. And maybe some recommended reading or audio on this topic.