THE COVENANT OF REDEMPTION! 
http://en.wikipedia.org/wiki/Covenant_theology
The Covenant of Redemption is the eternal agreement within the
Godhead in which the Father appointed the Son Jesus Christ by the
power of the Holy Spirit to redeem the elect from the guilt and
power of sin. God appointed Christ to live a life of perfect
obedience to the law and to die a penal, substitutionary,
sacrificial death as the covenantal representative for all who
trust in him.
http://www.thirdmill.org/answers/answer.asp/category/th/file/99786.qna
Question
What is the covenant of redemption? Did it involve all three divine
Persons? How did this doctrine develop historically? Isn't it a
post-Reformation doctrine? Can it be supported scripturally?
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Answer
The covenant of redemption is typically described as a
covenant between the Father and the Son rather
than as a covenant between all three members of the Godhead. This
is because the Bible presents the Son as the Redeemer, not the Holy
Spirit, and it presents the Father as the one who permits, commands
and accepts the Son's sacrifice. The Holy Spirit is critically
important to the salvation process, but the condition of redemption
was to be fulfilled by the Son in obligation to the Father, and the
Father was to reward the Son's obedience. This is the
essence of the covenant of redemption: a condition laid on
the Redeemer, and a reward promised to the Redeemer upon
fulfillment of the condition. Biblical texts supporting
this doctrine are quite extensive, but for a sampling of texts and
proofs see my article Biblical Soteriology: Limited Atonement, part
12.
I'm afraid I'm not aware of when the term "covenant of redemption"
first came into play. But the concept has been around much longer
than the term. In fact, I can't think of any old, mainstream system
of theology that would deny its basic ideas (though some newer
systems do), although the specific terms of that covenant are
disputed. Certainly the idea is present in Calvin (e.g. Institutes
2.12.4).
As in Calvin, I suspect that in very old systematic treatments the
idea would be addressed under the heading of the purpose of the
incarnation. One work that comes to mind is
Anselm's "Why God Became Man" (late 11th century).
Anselm certainly does not lay out the Reformed doctrine of the
covenant of redemption. But he does indicate that the Son of God
came to earth with the express purpose of redeeming mankind at the
will of the Father and by satisfying God's Law, and he assumes that
the Father assured the Son of success.