Stoicism places the human agent squarely within the physical world of causal chains. This raises the problem of responsibility. How can a person be held accountable for their actions if their actions were determined by natural laws? Can law courts operate if we refuse to accept free will?
In "New Stoicism" author Lawrence C.Becker proposes the following conditions for holding people responsible for their actions :
Agents are responsible for their acts if and only if :
1) They are aware of what they are doing.
2) They are aware of the causes of their actions.
3) They accent to acting in accord with the norms they recognize as their own.
4) They are aware of the causes of the accent.
5) They introduce new causal factors into the determination of their actions through the awareness of the causal conditions that shape it.
6) They are aware of this iterative self-transformative process and accent to this process.
The second condition - they are aware of the causes of their actions - is the most dubious. Is it possible to be aware of the all influences acting on you? But nevertheless Becker demonstrates how individuals can be held accountable for their actions within a deterministic framework or put other way, how people are responsible for their actions without accepting free will.
It's interesting to compare the Stoics version of free will and responsibility to the catholic version outlined in the Catholic Encyclopedia :
Spontaneous acts and desires are opposed to coaction or external compulsion, but they are not thereby morally free acts. They may still be the necessary outcome of the nature of the agent as, e.g. the actions of lower animals, of the insane, of young children, and many impulsive acts of mature life. The essential feature in free volition is the element of choice--the vis electiva, as St. Thomas calls it. There is a concomitant interrogative awareness in the form of the query "shall I acquiesce or shall I resist? Shall I do it or something else?", and the consequent acceptance or refusal, ratification or rejection, though either may be of varying degrees of completeness. It is this act of consent or approval, which converts a mere involuntary impulse or desire into a free volition and makes me accountable for it. A train of thought or volition deliberately initiated or acquiesced in, but afterward continued merely spontaneously without reflective advertence to our elective adoption of it, remains free in causa, and I am therefore responsible for it, though actually the process has passed into the department of merely spontaneous or automatic activity.
The outline above is very similar to the Stoic version. In both Christianity and Stoicism, a certain facility of the mind is considered capable to accepting or rejecting actions. This facility of the mind, or soul, or the accent, is unique to humans and not possessed by other animals.
The gap then between Stoic determinism and Christian free will is not as wide as it first appears.
See also: Self-realization and freedom