I love this verse. I love it because of many reasons, first and foremost being the fact that it places the Savior squarely in my corner come judgment. And we all know (or should know) that: one of Christ's main responsibilities will be to serve as our advocate to the Father.
But even though many of us know that, I think many of us misconstrue the meaning of one of the words in this verse, and completely miss another.
The word that is misconstrued is "advocate." I think that when most of us say or hear the word advocate, what springs to mind is a lawyer - someone who is in our employ, and who is paid to fight for us.
But an interesting thing about lawyers is that they are legally required to do the best they can by their clients... even if they don't believe their clients are in the right.* They may resign because they have a moral problem with representing the client, and they may not knowingly put forth false evidence under the law, but even if the client says, "I am absolutely guilty and should be given the maximum penalty for my crime," the lawyer's job is to fight for that guilty party, and (if possible) to get him/her off with no penalty whatsoever.
This, however, is not the Savior's role in advocating for us before the Father.
Which brings us to the next part: the word most people miss... or at least mis-read.
And that word is "cause." I think most people think that the Savior is going to plead our case. After all, that's what lawyers (again, what most of us think of when we think of advocates) do: they plead their client's case before the judge; again, casting the client in the best possible light to get the lightest possible sentence, guilty or no.
But, again, that is not what the Savior will do. He will not argue our case, for our case, if argued, would lead inevitably to our downfall and damnation. Our case, which would have to be argued truthfully before the great bar of the Lord, would include our sins and our shortcomings, our errors and our failures of heart. This would then automatically disqualify us from receiving Celestial glory, for "no unclean thing can dwell with God..."
Thankfully, however, the Savior will not argue our case... he will argue our cause. He will argue our cause.
The word cause has many definitions. One of them that I think particularly apt in this context is the following: "the welfare of a person or group, seen as a subject of concern." I like this. I like this because it converts the impossible idea of the Savior pleading successfully that we to be in the Celestial kingdom to the more workable principle and idea of him pleading for our welfare before the Father.
Of course, how is he going to do that? The answer lies (as it so often does) in reading the entirety of the thought, not just an excerpt:
"3 Listen to him who is the advocate with the Father, who is pleading your cause before him -
"4 Saying: Father, behold the sufferings and death of him who did no sin, in whom thou wast well pleased; behold the blood of thy Son which was shed, the blood of him whom thou gavest that thyself might be glorified;
"5 Wherefore, Father, spare these my brethren that believe on my name, that they may come unto me and have everlasting life."
The Savior will plead our cause - He will plead for our welfare, for our best interest, for the best possible avenue to happiness that will lie open to us.
And He will also plead a case. But instead of arguing like a lawyer in a court of law, who says, "Judge, this man is innocent," thus pleading the case of his client, the Savior will look to that great Judge of all and will say, "Father, this man [you and me] is guilty, but I am innocent, and have suffered that my friend might come unto me, and through me unto you."
In other words, the case that the Savior will argue will be His own: "I suffered, Father, and will You make My suffering vain?"
Of course, the Father would not, for such would not be just. And so we see justice and mercy merging and becoming, not two separate attributes, but one joint means of salvation and exaltation.
* This is a simplification, but it is essentially correct. And how do I know this? Because I am a lawyer. But don't hate me too much for that.