Romans 5:1 says, "having been justified by faith, we have peace with God". There are at least a couple of huge things to note:
1. this justification is a past event for the believer, not an ongoing process. It is done, and we now enjoy the benefits;
2. it is clear that the justifying is not done by us, but rather done for us. Self-justifying would be an ongoing process which would continually endanger "peace with God."
Therefore, efforts at self-justification are out of line. They are not productive, but rather destructive. They may feel good at the moment, but they do not result in "peace with God." They may get us out of trouble with offended parties for a brief time (though not usually), but self-justifications do not work with God. The Puritan, Henry Smith, says that "a sin is two sins when it is defended."
- Self-justification takes several forms. It can range from "it wasn't me" to "it wasn't my fault." We often give long lists of extenuating circumstances that explain or excuse our "bad" behavior, as though that makes it somehow less bad. It often involves rationalizing and blaming others. It is a regular refusal to take responsibility for our own sins and failures.
- Self-justification is abandoned as we confess our sins, and as we admit that we are sinners. God justifies the ungodly. That's me.
- Self-justification is abandoned as we understand the fallen world in which we live and of which we are a part and even a product (Romans 1).
- Self-justification is abandoned when we realize that even our religious and moral selves have inconsistencies and hypocrisies, let alone when we go a.w.o.l. and plunge into sin and filth (Romans 2).
- Self-justification is abandoned when we listen to ourselves talk, and realize that our words and attitudes are only a reflection of what is going on in our hearts (Romans 3).
- Self-justification is abandoned when we realize that our biggest task is not merely developing a skill to get out of trouble, but rather trusting God to do what we cannot do ourselves, justify sinners through the sin-bearing of Jesus on the cross (cf Rom 5:9).
God does not excuse sinners, he justifies them. And so we must not engage in strategies and schemes to excuse ourselves, but rather receive His justification by faith. There really is no excuse for us. But there is peace with God for us.