Will the Holy Spirit continue to dwell in a Christian who decided not to be a Christian anymore?
Your question, while simple and straightforward, is the tip of a mountain of underlying situations. Since I am not sure of the background of your question, I may not understand all your concerns.
The indwelling of the Holy Spirit is a special divine gift.
Christians can lose the special indwelling of the Holy Spirit through sanctifying grace by knowingly and willingly committing a violation of God's will in a serious/grave matter. Repentance and conversion restore the gift. It is God's grace through the working of the Holy Spirit that leads to repentance and restoration of the indwelling Holy Spirit through sanctifying grace.
Also, the Sacrament of Confirmation gives a special grace of the Holy Spirit and imparts a special seal on the soul. This seal continues. Possibly, one might think of that seal as leading to the grace for repentance and conversion after serious sin.
You could check The Catechism of the Catholic Church, available online at
The index has many references to the "Holy Spirit" and "Conversion" and "Penance and Reconciliation" and "Confirmation."
I would also recommend that you talk with a local Catholic priest. He would know additional circumstances in your community.
If you would like a further explanation or additional response from me, please feel free to write again.
My God bless you, Rachel, and keep you safe.
Please add your prayers to mine for a rich outpouring of God's love and grace upon you.