In this connection, We would draw the attention of Our own sons to the fact that the common good is something which affects the needs of the whole man, body and soul. That, then, is the sort of good which rulers of States must take suitable measure to ensure. They must respect the hierarchy of values, and aim at achieving the spiritual as well as the material prosperity of their subjects.It is not the case that there is any sense in which the State may aim at satisfying this spiritual aspect of the common good on its own terms. The State is an earthly construct, and to attempt such a thing would be to usurp a responsibility which properly belongs to the Church. I can imagine ways in which the State might become a help rather than a hindrance, however. One might be to guarantee freedom to worship (okay, that's a pretty obvious one) and freedom from the requirement to work on holy days of obligation.
Consisting, as he does, of body and immortal soul, man cannot in this mortal life satisfy his needs or attain perfect happiness. Thus, the measures that are taken to implement the common good must not jeopardize his eternal salvation; indeed, they must even help him to obtain it [PT §§57, 59].
I can readily imagine that doing this could raise questions as to how to fulfill this obligation while at the same time not running afoul of what the Holy Father had already said about political coercion and the common good. Coercion and reward were not flatly excluded there, though, so perhaps this duty of the State may be said to rest among those instances in which those means are to be permitted. In any case, it must surely be true that the State cannot act in such a way as to compel anyone to become a Christian, since this is by its nature something that must be done by an act of free will.