According to scholasticism, truth is God's WILL.
Perhaps the scholastic teacher most associated with this idea was John Duns Scotus (1266-1308). We might call him "John from Duns, the Scotsman." (That's what his name means - he was from the town of Duns on the Scottish border with England.) Voluntarism was the philosophical idea championed by John Duns Scotus -- from voluntas, Latin for "will." The teaching held that whatever God wills (desires) is always true. Therefore, truth and God's will are synonymous.
"Scholasticism" was the name applied to the theological and philosophical tradition of the "schoolmen" or university scholars in the Middle Ages. Later on, the reformer Martin Luther criticized some of the core teachings of the scholastics--the university teachers of theology. A central scholastic assertion was that if you "do what is in you" (fac quod in te est, as they said it in Latin), following that spark of God's will in you would gain you favor with God. Luther fought against scholastic philosophy with his emphasis on salvation coming solely by grace from God. The grace of God became a higher emphasis in Luther's theology, as opposed to the "will of God" emphasis in scholasticism.