Charles H. Spurgeon on Psalms 50:5

Made a covenant with me. Formerly soldiers used to take an oath not to flinch from their colours, but faithfully to cleave to their leaders; thus they called sacramentum militaire, a military oath; such an oath lies upon every Christian. It is so essential to the being of a saint, that they are described by this, Gather together unto me; those that have made a covenant with me. We are not Christians till we have subscribed this covenant, and that without any reservation. When we take upon us the profession of Christ's name, we enlist ourselves in his muster roll, and by it do promise that we will live and die with him in opposition to all his enemies ...He will not entertain us till we resign up ourselves freely to his disposal, that there may be no disputing with is commands afterwards, but, as one under his authority, go and come at his word. William Gurnall.