To stand surety for someone means that what has been bought cannot be repossessed. For whatever short fall there is, the surety pays. To be a surety of the better covenant which is the new covenant, means this. Jesus ensures that the promises in the covenant are made effective. Where there is no doubt that God will always keep his side of the covenant. It is our side of the covenant that needs a surety. An illustration of this would be when one enters into a covenant with a bank. The bank always works on the basis of surety. Either you must have collateral or someone must stand surety for you. That way the amount loaned cannot fail to be repaid and re posses ion can't take place. The only time it can be repossessed is when the surety fails. Christ however cannot fail. He is a sure surety. So Christ by his intercession as High Priest ensures our obedience to God. It is by Jesus working in us to obey, that we work out our salvation. The point is that those who are joined to Christ as partakers of the new covenant are part of the covenant of promise. So both sides are covered for Jesus stands as surety before God that we keep the human side of the covenant. Jesus ensures that Go d's promises are effective in what they have promised. It is unlike the Old Covenant which is conditional. The covenant of promise depends entirely upon God for its success. The old covenants success depended on mans obedience. But what happened it was a failure, for man rebelled and did not obey. The remnant were an exception. But now , Jesus ensures mans obedience, to those who draw near to God through Jesus. Indeed this is the point of the new covenant, to ensure the people of Go d's faithfulness. Praise God that we who trust in Christ, have such a great and mighty High Priest before the throne of God.