Complex sentence: The writer of the book of Hebrews indicates that Jesus, as the ultimate High Priest of Israel, went into the Heavenly temple to make atonement on behalf of all the blood sacrifices that had been made previously.
Split into simpler sentences: The writer of the book of Hebrews indicates that Jesus, as the ultimate High Priest of Israel, went into the Heavenly temple. There He made atonement on behalf of all the blood sacrifices that had been made previously.