Remarkably, much in contradistinction to the title, Darby deals mainly with baptism in this piece, and much with "household" baptism or the baptism of children of believers.
A sample of his reasoning is noteworthy, but many would disagree: “Believing then that the first man is entirely condemned and judicially ended in the death of Christ; that baptism is faith’s acceptance of this judgment; that God has given me children who are born, not in connection with the second Man, but in the nature of the first (‘that which is born of the flesh is flesh’); believing that these children, though thus born in sin, and in the nature of the first man (the ended man) are to be trained for God, brought up in the nurture and admonition of the Lord (He who has attained that Lordship by death), it is the privilege of faith to reckon such as judicially ended, to start from the death of Christ, remembering what God thinks of that death, that it is either salvation or judgment; that in it is displayed the righteousness of God, which is unto all, but which is only upon all them who believe.” (pp 10-11)