Objection 1. It would seem that charity is not the true form of the virtues. Because the form of a thing is either exemplar or essential. Now charity is not the exemplar form of the other virtues, since it would follow that the other virtues are of the same species as charity: nor is it the essential form of the other virtues, since then it would not be distinct from them. Therefore it is in no way the form of the virtues.
Objection 2. Further, charity is compared to the other virtues as their root and foundation, according to Ephesians 3:17: "Rooted and founded in charity." Now a root or foundation is not the form, but rather the matter of a thing, since it is the first part in the making. Therefore charity is not the form of the virtues.
Objection 3. Further, formal, final, and efficient causes do not coincide with one another (Phys. ii, 7). Now charity is called the end and the mother of the virtues. Therefore it should not be called their form.
On the contrary, Ambrose [Lombard, Sent. iii, D, 23 says that charity is the form of the virtues.
I answer that, In morals the form of an act is taken chiefly from the end. The reason of this is that the principal of moral acts is the will, whose object and form, so to speak, are the end. Now the form of an act always follows from a form of the agent. Consequently, in morals, that which gives an act its order to the end, must needs give the act its form. Now it is evident, in accordance with what has been said (7), that it is charity which directs the acts of all other virtues to the last end, and which, consequently, also gives the form to all other acts of virtue: and it is precisely in this sense that charity is called the form of the virtues, for these are called virtues in relation to "informed" acts.
Reply to Objection 1. Charity is called the form of the other virtues not as being their exemplar or their essential form, but rather by way of efficient cause, in so far as it sets the form on all, in the aforesaid manner.
Reply to Objection 2. Charity is compared to the foundation or root in so far as all other virtues draw their sustenance and nourishment therefrom, and not in the sense that the foundation and root have the character of a material cause.
Reply to Objection 3. Charity is said to be the end of other virtues, because it directs all other virtues to its own end. And since a mother is one who conceives within herself and by another, charity is called the mother of the other virtues, because, by commanding them, it conceives the acts of the other virtues, by the desire of the last end.
Source: Summa Theologica (New Advent)