Proponents of NFP often speak as if the reason that NFP is okay (and contraception is not) is that “NFP lets God decide when we have children.”
That’s a bit silly, though. Not only can God decide to do whatever he wants regardless of our family planning methods, but it isn’t even what the Church says about NFP.
Finally, it is for parents to take a thorough look at the matter and decide upon the number of their children. This is an obligation they take upon themselves, before their children already born, and before the community to which they belong—following the dictates of their own consciences informed by God’s law authentically interpreted, and bolstered by their trust in Him.
– Pope Paul VI, Populorum Progressio
Now, remember, this is the same Paul VI who issued Humanae Vitae, so he probably knows what he’s talking about.