Several views exist regarding the identity of the sons of God in Gen 6. These interpretations also affect how we should understand biblical references to supernatural beings including angels and foreign gods.
Sons of GodasDivine Beings
The sons of God may be divine beings (e.g., angels). If so, the sin in question was a transgression of the human realm by these heavenly beings. Their involvement with human women led to a widespread breakdown in morality and an increase in wickedness and corruption. The offspring of these unions, the nephilim (Gen 6:4), were considered quasi-divine and possessed unusual height (“giants”).
This was the dominant view among Jewish and Christian thinkers until after the fourth century ad, when Augustine championed an alternative (see below). It was also the exclusive view until the mid-second centuryad. …
Barry, John D., Michael S. Heiser, et al. Faithlife Study Bible. Bellingham, WA: Logos Bible Software, 2012. Print.