Of course, he doesn’t need to convert those already on his side but, rest assured satan can’t win, God is still in charge.
Seems to my that logically, Satan can’t infiltrate God’s church, as a matter of definition (which is why I don’t think he the adversary could contaminate the old and new testaments).
If it can be infiltrated and contaminated, to me that’s evidence it’s not God’s church. I’m Catholic, so that’s disconcerting if true.