God speaks His will to us in several ways, in Scripture, through the Church, and in personal prayer, and even through the people He puts closest to us.

It's instructive, always, to recall that we have a closed canon of Scripture as a by-product of decisions by the Church.   And it's through the Church that we have what should be a shared understanding among Catholics about such matters as the Trinity, grace, sin, the Sacraments, even the definition of love, including marriage and conjugal union.