Isaiah 26:1-8 reads like a Psalm.  It also functions like a Psalm since it is designed to urge us to sing its words.  The fact that it begins, “In that day this song will be sung…”, means it functions like all biblical prophecy–it shows us a picture of the future to urge true Believers to sing it now.  So, because of our relationship to God through faith in Christ and the power of the Spirit, we can now sing about our existence in this strong city (v. 1), we can evaluate whether we are citizens who are allowed to enter God’s city of salvation (vv. 2, 4-6, 8), and also enjoy city life characterized by “perfect peace” and a “level” path (cf. vv. 3, 7).  Our Savior makes it all possible for us as He experienced on the Cross the humiliation expressed in vv. 5-6 for us.  The songs of Isaiah, like so many Psalms, are designed to be experienced by Believers.  We sing these words to the extent that we experience the status they describe.  Prophecy moves us to adopt this lifestyle of trust and obedience (cf. v. 8).