S. Why is it necessary for it to impress another sign on the soul, doesn't the Character of Baptism suffice?
T. It is not without reason that a second sign is impressed on the soul, because through the first, a man is only known for a Christian and from the household of Christ, but on the other hand, by this second he is known as a soldier of Christ and therefore the mark of the General is placed on his soul, or rather, like the soldiers of this world that carry the banner of the Captain on their garments, or wear the insignia of their sergeants. But those who go to hell after they have received this Sacrament are especially ruined, to the extent that it seemed that they had taken up arms as soldiers of Christ by their own profession and by the Sacrament, but later rebelled hideously as traitors do.
[St. Robert Bellarmine, Doctrina Christiana: The Timeless Catechism of St. Robert Bellarmine, Grant, tr., Mediatrix Press (2016), pp. 162-163.]