In all of mankind since Adam, only one person has qualified to receive the inheritance of the promises that were made to Abraham—Christ.
We can see the requirements as early as Genesis 17:1, where God says to Abraham, "Walk before Me, and be perfect." Some Bibles translate it, "Be blameless," which means the same thing; "Be without sin." Christ, at the end of His life, was found to be blameless; therefore, He qualified to receive the promises. He met every condition of the Covenant, and then became the Inheritor.
Verse 29 is explaining that, if we are "in Christ" (in union with Him), then we become co-heirs with Him. We become co-inheritors with Him, if we have met the conditions the Bible gives: God has called us; we have unconditionally surrendered to God; we believe the gospel; we believe in the blood of Jesus Christ; we have been baptized; we have received the Holy Spirit; we have had hands laid on us. Then we also become "in Christ." The picture is as if we were part of Christ's body, and we are "in" Him. That is not actually what has occurred, but we are within the church.
Christ, being the Inheritor of the promises, then made out a will, as it were, prior to His death for the forgiveness of our sins. This will is also the New Covenant, which includes all the promises and blessings the Scriptures show us.
Christ had to die for a number of reasons. First of all, He was physical; and it is given to all men to die once (Hebrews 9:27). Another reason is that the wages of sin is death (Romans 6:23), and when our sins were placed on Him, He then came under the law and the law claimed its penalty—He died. Another reason is that He had to be transformed, glorified by means of a resurrection, because, as long as He was in the flesh, He could not inherit the promises either. One has to be eternal to inherit them; "flesh and blood cannot inherit the kingdom of God" (I Corinthians 15:50).