"Since we have now been justified by his blood, how much more shall we be saved from God's wrath through him! For if, when we were God's enemies, we were reconciled to him through the death of his Son, how much more, having been reconciled, shall we be saved through his life!"
This passage uses A fortiori logic to make an important argument. "A fortiori" logic is the concept that if you can do something greater, then you can do something lesser. A fortiori logic works like this: If your car has enough gasoline in its tank to travel 50 miles, then it also has enough gasoline to travel 5 miles. Paul is pointing out that God loved us enough to save us even when we were His enemies… so He must love us that much more once we become saved. And if He loves us more now that we're saved, and He loved us enough even when we were His enemies to make salvation available to us, why would He allow our sin to separate us from Him ever again? In other words, if our sin didn't stop God from providing salvation for us before we were saved, then why would our sin ever separate us from Him once we have been saved?
The same logic also applies to Romans 8:32, which says:
"He who did not spare His own Son, but delivered Him over for us all, how will He not also with Him freely give us all things?"
Paul is saying that God already did the greater thing… He sacrificed His only son who died to pay the penalty for all the sins of the world. So keeping us saved regardless of our failure is the easier, the lesser thing. If He already did the greater, then the lesser becomes a given.
Said another way, if our sins can cancel Christ's work on the cross once we're saved, then how did Christ's work on the cross ever cancel out any of our sins?