The Imputation of the Righteousness of Christ.
"For I say unto you, That except your righteousness shall exceed the righteousness of the scribes and Pharisees, ye shall in no case enter into the kingdom of heaven." Matthew 5:20.
This subject is so important I am creating a page dedicated to understanding it, aside from the page discussing the tri-synergist "way of salvation" proposed by Orthodoxy.
The imputation of the righteousness of Christ alone is denied by Catholicism, Eastern Orthodoxy, any such cult, sect or religion believing in works salvation, or priestcraft salvation (sacerdotalism), and even occasionally by Protestant and (so called) Evangelicals, I say "so called Evangelicals" here as having righteousness that is our own is a contradiction of the true concept of the "good news". Such heretical pseudo Evangelicals get mixed up about adding their own righteousness to salvation, by not understanding the simple way to refute Antinomianist type heresies such as "Once saved always saved no matter what you do" that is, that the imputation of the righteousness of Christ is for those who truly believe, and that true belief has the quality indicator of repentance. Repentance does not itself save (it is not Christ) but does indicate sincere not cynical belief, as there is a big difference between throwing away your salvation by a lack of repentance, and the idea repentance itself helped save you (as without grace repentance is not worthiness for salvation).