God.
Most likely God is mentioned in the Old Testament in what he said and did for His people.
The non-Hebrews, or gentiles, most often mentioned in the Old Testament are probably the Philistines. They were a sea-people, believed to have originated on the Greek islands, who arrived in the southern Levant around 1200 BCE.
Jesus is the One. The most frequently mentioned man in the New Testament of Bible.
God
Sheep.
Jerusalem.
His resurrection
His resurrection
His resurrection
Isaiah is quoted most in the New Testament and particularly Isaiah 53. While at first guess many would choose Isaiah...Isaiah might be the most quoted but not the most mentioned. Christ and the Apostles made frequent reference to Elijah. No other prophet is mentioned so frequently in the New Testament as Elijah. He occupies a highly significant place in the history of redemption. Also, since Elijah's ministry was limited to the kingdom of Israel, you will not find any accounts of Elijah in the Chronicles.
It is approximately 20% of the Bible. There is much of the Old Testament mentioned in the New Testament. Isaiah is quoted the most and referred to as the 'little Bible.'
Jesus, Paul & Peter