Why The Holy Spirit Is A Person Unto Herself

It seems to me that the personhood of the Holy Spirit stems from two central Christian convictions: that God and the Christian community are 'one' (pace the Gospel of John) and that the Christian is saved 'as a sinner'. How do I make sense of the fact that God in-dwells in me as I still remain convicted as a sinner? Some Christians have emphasized the regeneration of the believer, making the Christian into someone who stands perfected before God. This seems terribly false, to me. The other is to make this strict distinction between God and creation, and to completely remove the mystical element from religion. But the early Christian movement was clearly mystical in nature. Making a distinction between the Father and the Holy Spirit, and speaking of the Spirit as dwelling within the believer, I think, is part of the way early Christians solved this problem.