B. When and How Divine Worship is due

15. As to the second point [n.8], when and how this debt should be paid, I say that the command to do this act is an affirmative one; for it is reasonable that the intellectual creature be obligated to acknowledge his supreme Lord sometimes and to revere him, City of God 10.1; and this precept, like other affirmative precepts, is always binding but not binding at all times.

16. It can be supposed indeed that the obligation to perform this act belongs to the first precept of the first table [the Decalogue, Exodus 20.3, 5], ‘Thou shalt have no alien gods etc.’ This precept is not merely negative, prohibiting worship to be shown to anything it does not fit, but affirmative, that the true God should be held and worshiped as Lord. This is well expressed in Deuteronomy 6.13, and it is cited by the Savior, Matthew 4.10, ‘Thou shalt worship the Lord thy God and him only shalt thou serve.’ Here a positive precept about worship is set down, together also with exclusion of worship and service shown to another, just as also idolatry is forbidden in the first quotation, ‘Thou shalt not worship alien gods.’

17. Now it is impossible that this affirmative precept be always prevented from being carried out, as if an opportune time for doing what is commanded should never arise - as it might arise in the case of the precept ‘Honor they father and mother;’ for it is possible that an opportune time necessary for honoring parents should never arise, and so this precept could always be kept without transgression even if no act of the precept should ever be done; for the obligation to do the act is only for when an opportune time arises. But nothing could permanently prevent an opportune time arising for worshiping God; and so every adult is simply bound at some time to perform an act of this affirmative precept.

18. At the time of the Mosaic law a determinate time for carrying out this precept seems to have been given in the third commandment, ‘Keep holy the Sabbath day.’ But it was also a precept of the natural law, namely that at some determinate time a man should refrain from servile works; and the observation of the precept did not consist in this negative refraining alone, but in a positive act, namely in an act of ‘keeping holy’, that is, in a greater magnifying of God.

19. Now in the time of the Gospel law this worship, which is to be shown to God in the keeping holy of the Sabbath, is determined as to be done on the Lord’s day - and determined also as to the act by which it must be done, namely in the oblation of the supreme sacrifice, that is, the Eucharist, which the priest must offer for himself and for the people. And the people too make offering in this oblation, namely in a spiritual way, since they are bound ‘to hear the whole mass on the Lord’s day,’ according to the decree ‘Masses’ [Gratian p.3 d.1 ch.64]. And if some necessity excuse one from performing the act that the Church has determined, then one must fulfill it in some equivalent act, namely that (as regards the divine worship specifically deputed) some act be done that is referred directly to God and to reverence of him.

20. But as to whether anyone is bound to carry out this precept, namely to do some act of worship, at any other time besides the time determined - first in the law of nature and afterwards in the Mosaic law and now in the Gospel law on the Lord’s day - it seems that it is so, as on the great feast days during the year, as in the decree ‘Who on a solemn day,’ and ‘Of consecration’ and ‘One must pronounce’ [Gratian p.3 d.1 ch.66]. But if this is doubtful, yet it is certain that some worship at least on those days does seem it should be performed.