This essay is about two very different works, which nevertheless have one important characteristic in common. In both cases, a synthesis of opposites.is effected 

J. S. Bach, Motet No. 2, BWV226 "Der Geist hilft..." (1729)
CHORUS [Romans 8:26]
The Spirit helpeth our infirmities
for we know not what we should pray for, as we ought:
but the Spirit itself maketh intercession for us,
with ineffable sighing.
FUGUE [Romans 8:27]
And he that searcheth hearts, knoweth what is the mind of the Spirit,
because he maketh intercession for the saints, according to the will of God.
Thou heavenly aspiration,
Sweet consolation,
Help us now, so that joyfully and confidently
We may faithfully serve thee,
And not be deflected by sadness.
O Lord, prepare us through thy power,
And strengthen the reluctant flesh,
So that we may struggle valiantly,
And pass through death to life and to thee.

The first movement, the "Chorus," is divided into two halves, corresponding to the two halves of the biblical verse. The first half-verse, in 3/8 meter, is for double choir SATB-SATB, with eight independent voices. This half-verse is itself divided into two clauses of text, with different music for each. The two choirs, the eight voices, echo each other antiphonally. The melismas in sixteenth-notes are consistently on the word "Geist" (Spirit). 

The second half of the "Chorus," in 4/4 meter, is sung to the second half of the biblical verse. This half-verse is also divided into two clauses of text, with different music for each. The section begins with entrances of the melody for the first clause. After four entrances, bass 1 and bass 2 enter together, beginning a process of unification. The entrances are as follows:
m124-5 -- soprano 1
m126-7 -- soprano 2
m128-9 -- alto 1
m130-1 -- bass 2
m132-3 -- bass 1+2
m134-5 -- tenor 1+2 -- basses continue together
m136-7 -- alto 1+2 -- tenors and basses continue together
From m138 until the end of the "Chorus" the altos, tenors, and basses continue to sing together, so that the eight-voice double choir has become, for all practical purposes, a single five-voice choir SSATB. Until m137, some voices sang a melisma on the word "Seufzen" (sighing). From m138, soprano 1 and then soprano 2 sing the openng melody, while other voices sing the melisma and two-note "sighs." 

In the remaining two movements, the two choirs are completely united, and constitute a single choir SATB. The written score, however, continues with eight voices in two separate choirs, even though the music of the two choirs is identical. In performance, the choristers do not move toward each other. On the contrary, they remain separated and produce a stereophonic effect. 

The second movement is a double fugue. The two halves of the biblical verse are the texts for the two fugue subjects. The second subject includes a melisma on the word "Heiligen" (saints). The fugue begins with two separate expositions. In the first exposition, the first fugue subject is immediately presented as a canon. In the second exposition (m33ff) the second fugue subject is also presented as a canon. From m53 the two fugue subjects are sung simultaneously (each with its own text) revealing that together they constitute an invertible counterpoint. 

The concluding "Chorale" is typical of chorales harmonized by Bach. Especially moving are the two "Hallelujah"s at the end, the first ending with a plagal cadence, the second with an authentic cadence. 

To summarize: The music at the beginning of this motet is antiphonal; then it becomes polyphonic; then fugal, and finally homophonic. The two choirs become one. The eight independent voices become five; then four. Each of the fugue subjects is a canon at first, and then together they become one invertible counterpoint. Finally, the independent contrapuntal voices become the chords of a chorale. The motet ends with everyone singing "Hallelujah" together. 

Cesar Franck, Symphonic Variations, M46 (1885)
This work is in one movement, for piano solo with orchestra. In its form and style, however, it is nothing like a concerto. As the title implies, its form is derived from the traditional theme with variations. 

The work begins, in minor, with a dramatic dialog between the piano and the orchestra. The orchestra begins with extrovert dotted rhythms in unisons, and the piano answers with introvert chords. (Perhaps this was suggested by the second movement of Beethoven's Fourth Piano Concerto?) The thematic material mainly employs pairs of notes; those in the orchestra part are usually ascending, and those in the piano part are usually descending. 

This leads into the statement by the piano of a theme in minor (m100ff) which is followed by a set of variations. The theme employs the ascending pairs of notes that had previously characterized the orchestra. (The first two phrases had been precisely stated by the orchestra during the opening dialog, mm35-44). Now the theme is stated by the piano, in the form of a four-square 16-bar period, with two bars added in the last phrase. These added bars consist of two additional ascending pairs of notes, the first a major second and the second an augmented second. This irregularity is present in the variations as well.. 

The last variation is "molto piu lento, sostenuto, pianissimo." The cellos play the theme extremely slowly in major, accompanied by muted arpeggiated chords in the piano. Then, with the same texture and tempo and orchestration, the cellos play extremely slowly the melody in minor with descending pairs of notes that the piano had introduced earlier. 

This leads into the statement by the orchestra of another theme in major and "allegro non troppo" (m295ff) which is followed by a set of variations. This theme employs the descending pairs of notes that had previously characterized the piano. (The first two phrases had been precisely stated by the piano at the beginning of the opening dialog, but in minor, mm6-9). Now the theme is stated by the orchestra in major, in a 12-bar ternary A-B-A form. This form is present in the variations as well -- with a momentous exception. Two of the variations, the third and the last, are actually variations on the first theme, but in major, complete with the irregularity in the fourth phrase. This last variation leads into a vigorous coda based on the second theme. 

To summarize: The work begins by establishing the antithesis between orchestra and piano, and between ascending and descending pairs of notes -- and then proceeds to interchange them. The piano initiates a set of variations on the ascending-pair theme originated by the orchestra. After the central "Molto piu lento" section, the orchestra initiates a second set of variations on the descending-pair theme originated by the piano -- and minor changes to major, and the four-square period form changes to the ternary A-B-A form. The consummation of this synthesis of opposites occurs at the end, when variations on the first theme occur in the course of the second set of variations. It is no accident that the work concludes with two tonic chords, followed by a pause, followed by one single tonic chord. 

These two composers, dissimilar as they were, had one attribute in common: both were devout Christians. It is plausible to assume that their having composed music in which diverse elements merge into a jubilant unity emanated from their monotheistic faith. In this they were very different from the self-appointed spokesmen of God, who never tire of inventing multitudes of supernatural objects of worship: the Trinity, and the Holy Family, and hierarchies of angels and devils, and spirits and prophets and martyrs and saints. It has been well said, that the only good that religion has bestowed upon humanity is the art and the architecture and the music. At any rate, music like this lends credibility to the thesis that the monotheism of these two composers was purer and nobler than that of the bean-counting, superstition-ridden theologians.