The separation between God and man was symbolized by a large, thick curtain in the temple which separated the Holy Place from the Most Holy Place. The Ark of the Covenant, with the Mercy Seat of God was behind the curtain. Nobody but the High Priest was allowed behind the curtain, and he was only allowed into the presence of God once a year, on the Day of Atonement.

And even then, the High Priest was only allowed in the Most Holy Place after the proper cleansing ritual.