At the moment, I feel rather mixed about the issue, to be honest. My Christian upbringing had an awareness of an embodied and personal being called Satan, the Devil, but it didn't have a huge emphasis on the nature or impact on a day-to-day level of such a being. I think there was an assumed approach that to give credence to events as the Devil's handywork was to assign too much to Satan than was necessary and to give Satan a near-Divine status which was unwarranted. So the issue was just not talked about, and everyone assumed everyone else knew what was being talked about when they spoke about the Devil.

Later in life, as I've become more suspicious of the heritage I grew up within, the issue has become mush more muddy. There's been several areas that have impinged on my understanding. One has been the work of Walter Wink in his influential Powers series, and especially how he has drawn on an applied the work of Girard in his reflection on violence, domination and the Powers.

Wink's conception of the Powers that the Bible talks of is focussed on the human arrangements of power and argues that the New Testament language presupposes attention on the human incumbent-in-office. So, for him, the Powers are the inner aspect of material reality, the aura or dynamic of a group of people (like a corporation or supporters at a sports event). So, the spiritual beings we are familiar with referring to he argues are not separate or etherial entities but the patterning of physical things, the inner essence, the gestalt, of an institution or state or system, the psychic or spiritual power emanated by organisations or individuals that are not idolized but conserve the self-consistency of each level of reality (Naming the Powers: The Language of Power in the New Testament [Philadelphia: Fortress Press, 1984], p104ff). Wink argues for an understanding of the Greek word kosmos, usually interpreted 'world' (as in being in 'the world' but not of 'the world'), as 'domination system' (cf. 1 John 2:15-17) and that it 'refers to the human sociological realm that exists in estrangement from God' (Engaging the Powers: Discernment and Resistance in a World of Domination [Minneapolis: Fortress Press, 1992], p51). He then argues that Satan is the presiding spirit of the Domination System (cf John 14:30-31).

Though Wink's hermeneutic can be questioned in several places, his overall scheme makes a lot of sense to me.

I've also been thinking more recently about the Jewish monotheistic conception of God (Cf. the Shema: 'Hear, O Israel: The LORD our God, the LORD is one.' Deuteronomy 6:4). If the God of Jesus is the same as the God of Israel, then there is no room for a quasi-Divine opposite to the all-powerful, all-good theistic God we assume is the God of Israel. In the Jewish conception then, there is only one God, YHWH, and all things come from God, good and evil. If I'm going to accept that Jesus was fully Jewish, which I do, then I have to accept the God of the Shema whom Jesus spoke of.

I know that's not a fully-formed response, but that's kinda where I've got to at the moment. Maybe I'll get further through this conversation.