In this covenant God the Father and the Son made an agreement with regard to the elect. This covenant was made before the universe was created and it consisted of the Father promising to bring to the Son all whom the Father had given Him (John 6:39;17:2,9,24), to send the Son to be the representative of the people (John 3:16; Rom. 5:18-19), to prepare a body for the Son (Col. 2:9; Heb. 10:5), and to give the Son all authority in heaven and earth (Matt. 28:18).
The Son would become man (Col. 2:9; 1 Tim. 2:5), become for a while lower than the angels (Heb. 2:7), and be found under the Law (Gal. 4:4-5). The Son would die for the sins of the world (1 John 2:2; 1 Pet. 2:24) and the Father would raise the Son from the Dead (Psalm 2).
The Holy Spirit would empower Jesus to do the will of the Father and Christ’s ministry on earth (Matt. 3:16; Luke 4:1,18; John 3:34), and to apply the redemptive work of Christ to Christians (John 14:16-17,26; Acts 1:8; 2:17-18).
Normally, covenants are made between those of different authority levels such as a king and a subject. But in the Eternal Covenant, all the parties involved (the Father, the Son, and the Holy Spirit) are equals.