I have little to add to what Deacon Peter pointed out. EO and OO are basically holding almost the same believes. We believe that the holy spirit never depart from the person after baptism and anointment in the sacrament of confirmation, even if he denies Christ. Therefore, apostates who want to come back to the Christian faith are not baptized again, in line with the Church Fathers teachings after the persecution period under Pagan emperors.
No sin can cast out the Holy Spirit, we believe it is just extinguished slowly. Repenting, Confession and Communion will make the Holy Spirit glow again.
To read more about the Holy Spirit in the OO Tradition, I advise to read the books by His Holiness Pope Shenouda, Pope of Alexandria, His Grace The Late Bishop Youaness about the Holy Spirit. Problem is that these books are in Arabic and I doubt that they are translated. The generation who immigrated to the US and the first generation born in North America are working on translating our heritage to English so we can contribute to the growth of the Orthodox faith in the West.
One quick question:
I think the following quote from Arch. Eusebius Stephanou is at odds with modern-day RC Pneumatology because the RCC teaches that mortal sin drives out the Spirit
Do RC really believe that the Holy Spirit can be driven out by sin ? I am just surprised and I find it hard to believe and therefore wanted to confirm that I understood the mentioned phrase right. If so, what is a mortal sin ?