Ultimately God always has total control over every area of our lives, no matter what we do, simply because He is God and therefore omnipotent. If He chooses (as He once did) even a virgin can conceive and bear a child. But God has given each of us free will and thereby the choice to cooperate with His plan for us or not. Couples who use NFP in a morally correct manner do so in an attempt to cooperate with Gods plan for the number and spacing of their children not restrict His authority over their families.
As any parent will tell you there is a great deal more to being a good Christian parent that just popping em out. The Church recognizes that the education and upbringing of each child is a tremendous responsibility and that there are limits physical, material, psychological and social to the number of children many couples can raise well. The Church, therefore, does not have any specific teaching on the ideal family size. All married couples are called to be both generous and responsible in their acceptance of children, but the exact number and spacing of those children is a matter for each couple to discern privately. Granted, in this day and age, the temptation to forgo generosity in favor of responsibility is usually stronger, but it is not somehow better to fail to be responsible in the use of our procreative powers than to fail to be generous.
Pope Paul VI clearly explained this need to cooperate with Gods dual call to generosity and responsibility in Humanae Vitae: With regard to physical, economic, psychological and social conditions, responsible parenthood is exercised by those who prudently and generously decide to have more children, and by those who, for serious reasons and with due respect to moral precepts, decide not to have additional children for either a certain or an indefinite period of time.
Responsible parenthood, as we use the term here, has one further essential aspect of paramount importance. It concerns the objective moral order which was established by God, and of which a right conscience is the true interpreter. In a word, the exercise of responsible parenthood requires that husband and wife, keeping a right order of priorities, recognize their own duties toward God, themselves, their families and human society.
From this it follows that they are not free to act as they choose in the service of transmitting life, as if it were wholly up to them to decide what is the right course to follow. On the contrary, they are bound to ensure that what they do corresponds to the will of God the Creator. The very nature of marriage and its use makes His will clear, while the constant teaching of the Church spells it out.
Here, too, ice cream provides a good analogy. The act of eating ice cream in and of itself is not immoral in the least. Humans are designed to enjoy sweets, even newborn babies strongly prefer sweet tasting liquids and human breast milk is remarkably sweet. But there are times when the good of enjoying dessert ought to be foregone for the sake of a greater good. If I am severely obese, have high cholesterol and have been warned repeatedly that I will almost certainly have a heart attack and be unable to care for my children if I do not modify my diet, to go ahead and eat as much ice cream as I desire whenever I desire is to fail to exercise the virtues of both prudence and temperance and is therefore morally wrong.
Likewise, if a couple has discerned through prayer, reflection and discussion that because they have a serious reason to avoid pregnancy it is not Gods desire for them to conceive again at present, then they are called to cooperate with God and use morally licit means (NFP) to avoid conceiving. In doing so they no more usurp Gods authority over their family than they do when they seek employment to provide income to meet their material needs rather than waiting for God to send them manna from heaven or educate their children rather than waiting for each of them to be divinely inspired.
from Envoy Magazine
