Objection 1. It would seem unfitting to distinguish three degrees of charity, beginning, progress, and perfection. For there are many degrees between the beginning of charity and its ultimate perfection. Therefore it is not right to put only one.
Objection 2. Further, charity begins to progress as soon as it begins to be. Therefore we ought not to distinguish between charity as progressing and as beginning.
Objection 3. Further, in this world, however perfect a man's charity may be, it can increase, as stated above (Article 7). Now for charity to increase is to progress. Therefore perfect charity ought not to be distinguished from progressing charity: and so the aforesaid degrees are unsuitably assigned to charity.
On the contrary, Augustine says (In prim. canon. Joan. Tract. v) "As soon as charity is born it takes food," which refers to beginners, "after taking food, it waxes strong," which refers to those who are progressing, "and when it has become strong it is perfected," which refers to the perfect. Therefore there are three degrees of charity.
I answer that, The spiritual increase of charity may be considered in respect of a certain likeness to the growth of the human body. For although this latter growth may be divided into many parts, yet it has certain fixed divisions according to those particular actions or pursuits to which man is brought by this same growth. Thus we speak of a man being an infant until he has the use of reason, after which we distinguish another state of man wherein he begins to speak and to use his reason, while there is again a third state, that of puberty when he begins to acquire the power of generation, and so on until he arrives at perfection.
In like manner the divers degrees of charity are distinguished according to the different pursuits to which man is brought by the increase of charity. For at first it is incumbent on man to occupy himself chiefly with avoiding sin and resisting his concupiscences, which move him in opposition to charity: this concerns beginners, in whom charity has to be fed or fostered lest it be destroyed: in the second place man's chief pursuit is to aim at progress in good, and this is the pursuit of the proficient, whose chief aim is to strengthen their charity by adding to it: while man's third pursuit is to aim chiefly at union with and enjoyment of God: this belongs to the perfect who "desire to be dissolved and to be with Christ."
In like manner we observe in local motion that at first there is withdrawal from one term, then approach to the other term, and thirdly, rest in this term.
Reply to Objection 1. All these distinct degrees which can be discerned in the increase of charity, are comprised in the aforesaid three, even as every division of continuous things is included in these three--the beginning, the middle, and the end, as the Philosopher states (De Coelo i, 1).
Reply to Objection 2. Although those who are beginners in charity may progress, yet the chief care that besets them is to resist the sins which disturb them by their onslaught. Afterwards, however, when they come to feel this onslaught less, they begin to tend to perfection with greater security; yet with one hand doing the work, and with the other holding the sword as related in Nehemiah 4:17 about those who built up Jerusalem.
Reply to Objection 3. Even the perfect make progress in charity: yet this is not their chief care, but their aim is principally directed towards union with God. And though both the beginner and the proficient seek this, yet their solicitude is chiefly about other things, with the beginner, about avoiding sin, with the proficient, about progressing in virtue.
Source: Summa Theologica (New Advent)