Our revisers retained the underlying principles. By spreading the course over 30 days they made it possible to use it all. They retained the 95th Psalm as the first Psalm of every day; and also the principle of having two daily Services at which Psalms occupied an important place.
There are Special Psalms for six days in the year—the four great Festivals, Christmas, Easter, Ascension, and Whitsun Day, and the two great prayer-days, Ash-Wednesday and Good Friday. The Preface explains that these Special Psalms are to be sung instead of the ordinary Psalms on those days; and authorises the use of Special Psalms approved by the Ordinary on other days.
In using the Book of Psalms as a book of worship we must remember what was said of the Intention of our minds in respect to parts of the Services. There are many Psalms which supply us with the best Prayers in trouble, penitence or any anxiety. But when using them in these Services our Intention is not Prayer but Praise, and the thought of God must inspire our devotions.
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It will often help us if we remember that God's Righteousness is infinite, as well as His Mercy. It is impossible for man in his present state to reconcile perfect Righteousness and perfect Mercy: for Righteousness will have nothing to do with sin, while Mercy forgives it. These two characteristics of God are revealed to us through Christ in Whom Righteousness and Peace are united; cf. Ps. lxxxv.
The Psalms, composed by various people at different times, very frequently are the utterances of men in trouble: and they often sketch the thoughts or actions of the Ideal Man, in one or other of the four characters which answer to God's Righteousness and God's Mercy. For, in response to God's Righteousness, man must be (1) perfectly penitent, and (2) in imitation of God, must detest sin: in imitation of God, (3) he must be perfectly forgiving, and in response to God's mercy, (4) he must have trust and peace. The Psalmists exhibit human nature at its best, but it is human nature all the time—human nature finding God and associating itself with the Ideal Man.
Thus the Psalms often rise to the conception of the Messiah; and, even when that is not their thought, they proceed from other thoughts to Rest in God and Praise of His Holy Name.
The most difficult Psalms for worship are those which regard sin with horror, but express the horror without mercy. Man is unable to hold the two qualities of Righteousness and Mercy simultaneously. We find it difficult in these days to detest sin because we are learning the quality of mercy.
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Much of the poetic force of these songs depends on the local incidents of Israel's history and the scenery of Jerusalem and the Holy Land. While we use the words, we must also use our imaginations to transfer the great thoughts to our own experience: for those local colours are the clothing of thoughts which belong to all men in their relation to God.