'This song' is to be interpreted as a song, not with the cold-blooded accuracy proper to a scientific treatise. The logic of emotion is as sound as that of cool intellect, but it has its own laws and links of connection. First, the song sets in sharp contrast the two cities, describing, in verses 1-4, the city of God, its strength defences, conditions of citizenship, and the peace which reigns within its walls; and in verses 5 and 6 the fall and utter ruin of the robber city, its antagonist Jerusalem, on its rocky peninsula, supplies the form of Isaiah's thought; but it is only a symbol of the true city of God, the stable, invisible, but most real, polity and order of things to which men, even while wandering lonely and pilgrims, do come, if they will. It is possible even here and now to have our citizenship in the heavens, and to feel that we belong to a great community beyond the sea of time, though our feet have never trodden its golden pavements, nor our eyes seen its happy glories. In one aspect, it is ideal, but in truth it is more real than the intrusive and false things of this fleeting present, which call themselves realities. 'The things which are' are the things above. The things here are but shows and shadows. The city's walls are salvation. There is no need to name the architect of these fortifications. One hand only can pile their strength. God appoints salvation in lieu of all visible defences. Whom He purposes to save are saved. Whom He wills to keep safe are kept safe. They who can shelter behind that strong defence need no other. Weak, sense-governed hearts may crave something more palpable, but they do not really need it. A parapet on an Alpine road gives no real security, but only satisfies imagination. The sky needs no pillars to hold it up. Then an unknown voice breaks in upon the song, calling on unnamed attendants to fling wide the gates. The city is conceived of as empty; its destined inhabitants must have certain qualifications. They must be righteous, and must 'keep faithfulness' being true to the God who is 'faithful and true' in all His relations. None but the righteous can dwell in conscious citizenship with the Unseen while here, and none but the righteous can enter through the gates into the city. That requirement is founded in the very nature of the case, and is as emphatically proclaimed by the gospel as by the prophet. But the gospel tells more articulately than he was enlightened to do, how righteousness is to be won. The last vision of the Apocalypse, which is so like this song in its central idea, tells us of the fall of Babylon, of the descent to earth of the New Jerusalem, and leaves as its last message the great saying, 'Blessed are they that wash their robes that they may ... enter in through the gate into the city.'