To fully understand it you must consider the initial application. They were being called out of Babylon to restore Jerusalem. Many were complacent in Babylon after 70 years of captivity they had lived through extreme oppressions through changes of leadership and acceptance until they had all but been assimilated into Babylonian society and many were experiencing prosperity, position and luxury.
They were loosing their identity and were in danger of abandoning their religion and yet there was a remnant that hungered and looked for righteousness. This is the prophetic call to awake to righteousness and come out of Babylon and go restore Jerusalem and the temple. God was going to miraculously deliver them by having Cyrus of the Persian decree that they were free to go home and rebuild their city and God was calling them to awaken to the concept.
Some were thinking they were too weak and too few to do such an impossible task. There are three calls to Awake, Awake in the two chapters. They are called to awake by God, then they respond with an awakening, and put God in remembrance of His deeds of old when He took them out of Egypt, asking him to Awake, Awake, and then God calls them to Awake, Awake and promises them deliverance from Babylon.
Then, after meditating on all the details and the back and forth of the Awake, Awakes, immediate application related to the deliverance of Babylon and to come out from among her and go into the land of promise and rebuild we can properly apply it to the prophesies concerning the future Gospel of Jesus Christ and how He would deliver them in a salvation that had more than the land of Israel in mind.
"How beautiful on the mountains are the feet of them" applied to those who were messengers to declare that the decree had gone out from Cyrus to let the people return to Israel.. Make a highway in the desert etc... they were going to come in and nothing was going to be able to stop them because God was going to make it happen...But also Paul applied "how beautiful are the feet of them" to the preachers of the Gospel which is a far greater mission we have. So we are also telling them to Come out from the World and Follow Christ to restore the real everlasting Kingdom and not that dirt in the middle east. But the Babylonian exile and deliverance is the shadow story that is necessary to know how to properly apply the prophetic substance story of Christ and the everlasting kingdom.
So yes the garment of salvation has to do with Christ and faith in the Gospel. The Whole armor of God all has Faith in the Gospel at its' root. The helmet of salvation means that your mind is renewed to the faith about your salvation and you don't let the devil lie to you about not being forgiven. We are those whose feet are beautiful because we are the messengers come to tell others that the decree has gone forth and the day of deliverance from the powers of darkness is here.
And there are future applications to these promises because all the world will be filled with the salvation of God and sin will be no more.
There was an application related to coming out of Babylon and coming back to restore Israel in 538BC
There was an application that applied to the first coming of Jesus and the Gospel now being proclaimed.
There is an application that applies to the many promises that we look forward to concerning the final perfection to come.
