Romans 5:1-11 is an amazing passage.
In the context, Paul has taught about the problem of sin (1:18-3:20), is in the middle of explaining the provision of salvation (3:21-5:21) and will turn his attention to the power for sanctification (6-8).
As he develops the provision of salvation, Paul lays out what God has done through Christ (3:21-31) to enable sinful man to have a relationship with Him by faith (chapter 4).
That’s where chapter 5 picks up–“Therefore, since we have been justified by faith, we have peace with God.”
The present reality of a past action.
When we place our faith in Christ’s cross work, God declares us to be in a right relationship with Him (the removal of guilt, the bestowal of positional righteousness) and replaces hostility with peace.
From this first verse, we can see that we can know for certain that we are justified (“have been justified”). We can know for certain we are saved.
And that’s just verse one of this great passage.