How do we know God's forgiveness is perfect?

In Isaiah 40, when it declares that her iniquity is pardoned, it signifies complete and unconditional forgiveness provided through Jesus Christ's atoning work. The accomplished atonement means that all our sins, past, present, and future, are fully paid for by Christ on the cross, who bore the wrath of God that we deserved. This perfect imputation of righteousness means that nothing can be held against us, because Christ’s work was sufficient to secure our complete forgiveness before God. As a result, believers can rest in the assurance that they are eternally forgiven.