Their reason: nothing is future, absolutely or conditionally, except by a decree predetermining the will
In their view, nothing becomes future, whether outright or on a condition, unless a decree actually comes first in God and gives it that status. Through such a decree God settles that what is called future will be brought about by him, or at the least allowed.
It follows that, until God's will has decreed to predetermine the human will toward one of two contradictory or contrary options, no one can know what a particular person will do. Nor can anyone know what he would do in such circumstances; all that can be known is what he is able to do. Before such a decree the thing is not yet future and not yet non-future. It is simply possible, and open either way.
Latin
XX. Putant enim nihil esse ullo modo futurum, vel absolute, vel sub conditione, nisi virtute alicujus decreti, quod actu in Deo praecedat, & quo Deus constituerit id quod futurum dicitur, vel efficere, vel saltem permittere; adeoque ante decretum divinae voluntatis praedeterminantis voluntatem ad alteram partem contradictionis, vel contrarietatis, non esse scibile quid in particulari faciet homo, vel quid facturus esset, si constitueretur in talibus circumstantiis; sed solum quid posset facere: idque quoniam ante tale decretum, res nec est futura, neque non futura, sed mere possibilis, & indifferens ad hoc, ut futura sit, vel non futura sit.