"For I am not aware of anything against myself, but I am not thereby acquitted. It is the Lord who judges me." (ESV)
A clean conscience is not sufficient evidence of one who has been made righteous. Paul recognized that even though he was consciously aware of no sin in his life, he was not therefore justified in claiming to be righteous. Having a clear conscience is good and necessary, but it is not sufficient to assure righteousness. Neither is it by the inquiry or testimony of others. We may be cleared by a jury of our peers, our spouse, our church, the whole world and even our own conscience, but this does not assure innocence. The only way Paul can claim, "I am free from accusation and guilt" is because he has been examined by the Lord. Unfortunately, some of the original emphasis is lost in the English translation (here comes the Greek lesson).
Note Colwell's Rule and Colwell's Corollary in play here:
ὁ δὲ ἀνακρίνων με κύριος έστιν (but the one who judges me is Lord)
In the Greek text, the word order and lack of article before κύριος (Lord) shows that emphasis is being placed on the quality of κύριος. It is the examination by one who is Lord that can ultimately vindicate or condemn. No other authority can do either, not even our own consciences. What a relief to know that my hope rests in the LORD Jesus, and not my own fickle conscience.
 Colwell's Rule states that "definite prediacte nouns which precede the verb usually lack the article." (Wallace, Greek Grammar, 257)
Colwell's Corollary states that when "an anarthourse pre-verbal predicate nomonative is normally qualitative, sometimes definite, and only rarely indefinite." (Wallace, 262)
The "Rule" simply tells us that since κύριος comes before the verb ("is") and that, if it is definite, it is likely to not have an article (which it doesn't). Okay.... So what? So, we are not required to translate is as indefinite ("a Lord"). Okay good,
but keep reading!
The "Corollary" to the rule tells us κύριος is most likely qualitative, not definite or indefinite. That is, the emphasis is on the quality of being "Lord." An expanded translation would look like this:
"I am judged by one who has the quality of being Lord"
It is Jesus' status as Lord that makes His acquittal meaningful. Since there is no Lord other than Jesus, only He can declare our innocence or pronounce our guilt.