It's not nice to distract me with a question right in one of the areas in which I have a great deal of interest when I have a conference presentation to prepare.
Let's get this out up front: this answer will be confined to western Europe from Antiquity to the later Middle Ages.
Probably the greatest philosophical influence on the early Christian conception of the will is Stoicism. The Stoic idea is best described as a boat - you can go wherever you want, but you're still going to get to your destination. Actually, it makes an appearance in Tom Stoppard's great film/play Rosencrantz and Guildenstern are Dead - check that out if you like this sort of thing.
When it comes to early Western theology, the name is Augustine, and Augustine pretty fully adopted this idea. Or, he did eventually. Augustine's theology on the will evolved over time towards a more strictly predestinarian stand as a reaction to the heresy of Pelagius, who insisted that grace was not necessary for humans to achieve salvation. In the end Augustine insisted that divine grace was irresistible, although scholars still debate as to whether he really thought humans were predestined to salvation or damnation.
Augustine's ideas on the irresistibility of grace were formally rejected by the Council of Orange in 529.
Whatever Augustine thought, and whatever Church councils said, many people read him as explicitly predestinarian, most noticeably the 9th century monk Gotteschalk. In what is known as the Predestinarian Controversy, Gotteschalk's writings were condemned as heretical for asserting absolute predestination, but the official response drawn up by the theologian Scotius Eirugena and the powerful archbishop Hincmar of Reims was similarly condemned as being too Pelagian.
This problem with Augustine basically festered on the back burner until the writings of the theologian and archbishop of Canterbury Anselm of Bec (d.1109). Anselm's theology of the will is moderately complex, but the essence of it is that grace is freely given to everyone, but to be given a thing must be offered and accepted. This is our freedom, which Anselm calls "free choice" (liberum arbitrium) and is distinct from the will (voluntas).
For Anselm, the will itself is not free - it is constrained and imprisoned by sin. As he put it
The will wishes that it would will what it ought to will (velle se debere velle quod vult).
This is perhaps the hardest part of understanding the medieval idea of the will. The inability to sin was a freedom. Liberated from that burden, the will was free to do what it wanted, i.e. seek out and follow the Divine. As the great medievalist Gerd Tellenbach put it,
To the medieval mind, true freedom was in complete subservience to the divine.
Anselm's solution to the problems inherent in Augustine was not totally perfect, and there were several changes made over time, but its utility was recognized. I've previously made a post on its effect on medieval spirituality here.
Some reading:
Sweeney, Eileen C. Anselm of Canterbury and the Desire for the Word. Washington, D.C: Catholic University of America Press, 2012.
Rogers, Katherin A. Anselm on Freedom. Oxford ; New York: Oxford University Press, 2008.
Tellenbach, Gerd. Church, State, and Christian Society at the Time of the Investiture Contest. Toronto ; Buffalo: University of Toronto Press, 1991.