Suretyship imports that the obligation be free and voluntary, for the Law forces none to be a Surety, or to engage for others. My brethren, though God did choose Jesus Christ to be the Surety of this Covenant for us, yet Christ, as a most free and voluntary act on His part, undertook that office. For that Law we had broken, laid no obligation on him, nor was He under any necessity of Nature to undertake herein, because He was the Son of God, but this choice is ascribed wholly to His infinite love and goodness. It is a Sovereign act of His own free grace to undertake for man, and not for angels, and also only for some of the lost Sons of Adam, and not for all. “No man takes my life from me, but I lay it down freely; I have power to lay it down, and I have power to take it up again,” John 10:18. “Lo, I come in the volume of the Book, it is written of me to do thy will, O God,” Hebrews 10:5, 7.
Benjamin Keach, 1689 (as posted on Shreveport Grace Church bulletin - 29 October 2007)