The old covenant: This was weak and bound to fail, because it was between fallen mankind and a Holy God. The fallen man could not, nor ever could keep the covenant. The covenant broke down almost straight away and mankind fell into ruin.
The new covenant actually existed before the old covenant: This was a strong and Holy bond, because it was between the three persons of the Holy Trinity, and depended on them only to keep it and sustain it.
The first covenant was then the one made between the persons of the Godhead, so that sinful man could be redeemed and sanctified and brought safely into heaven itself. The covenant made with man, was seen to be useless, as it could not be upheld by the weak (dead) party.
God therefore demonstrated the weakness of sinful man and the glory of His own Holy strength and power to buy back for slavery and death a people for his name.
God the father willed this event, the Son gave himself as a ransom for many and the Spirit calls those who are being saved and indwelling them and keeping them until the day of final redemption. Thus the Holy covenant is fulfilled and sinful man can now enter in with true confidence that God is able to save to the uttermost, those that come to God through Him.
