Greek: Ἐὰν ἀγαπᾶτέ με, τὰς ἐντολὰς τὰς ἐμὰς τηρήσετε·
My Translation: If y'all love me, y'all will observe my commandments.
KJV: If ye love me, keep my commandments.
My translations are purposefully stretched and should not be viewed as more accurate than the KJV translation unless I say so in the post. I'm trying to show the range lying between the original Greek text and the English.
When I mentioned that the Jesus of John's gospel likes to talk, I'm not kidding! John 14 begins a discourse from Jesus to his disciples that lasts for several chapters without any real interruption. The context is the famous Last Supper, mentioned in all four gospels and also in the genuine letters of Paul. Jesus is here giving his last sermon to his disciples, preparing them for his death and (in the Gospel of John, at least) for how his death will bring the Kingdom of God fully to them. The broader context is difficult to summarize; I'd recommend that you go and read John 14 for yourself. The best I can say is that Jesus is talking about his commandments and about love.
Frankly, I'm going to commit a huge blasphemy here and say that I don't really find this scripture to be engaging at all. I'm dreading this write-up because of the boredom this verse inspires to me. I'm wanting to talk about how it is emblematic of the Mormon doctrine of the importance of works, but frankly I think the case for this to be made is weak. Jesus is here making a command that his disciples show their love of Jesus by following what he had told them to do. He is not saying that they must do so or else they will not enter the kingdom of God. He is not saying they must do so or suffer wrath. He's saying that their love of him should lead to their doing what he said to do. A little later, in verse 21, he indicates that the way to show love for Jesus is by keeping his commandments. They're not two things that lead from one to another, they're equivalent things. So does this really have much to do with salvation by either grace or by works? Not really, I'd argue, without reading a lot into the text.
A better question here would be, "What are the commandments of Jesus?" The typical LDS answer (indeed, the answer of most biblical inerrantists as well) would be that anywhere in the Bible where Jesus says to do something, then that is one of his commandments. Is this the case?
John's Jesus is remarkably ambivalent about the Jewish law (unlike Matthew's Jesus, with whom we've spent the most time so far). If we look through this chapters-long discourse, the answer becomes clear when we look at the next chapter (remember that there are no real chapter divisions in John's original work; this is all the same thing). John 15:10-14 says,
If you obey my commandments, you will remain in my love, just as I have obeyed my Father's commandments and remain in his love. I have told you these things so that my joy may be in you, and your joy maybe complete. My commandment is this---to love one another just as I have love you. No one has greater love than this---that one lays down his life for his friends. You are my friends if you do what I command you.
And just a bit later in verse 17:
This I command you - to love one another.
I think it's very difficult to argue that the commandments referred to in this scripture mastery verse are anything more than directly linked with the explicit commands that Jesus gives to his disciples in the next chapter: that they love each other enough to die for each other. Sure, we can find many more commandments of Jesus in the other gospels, but the gospels were not written to be read together. They are not friendly sources towards each other. Each gospel is a unique piece of literature that attempts to stand alone in their depiction of who Jesus was and what his message was. To honestly discern what the author of the gospel was referring to beyond the command to love one another would best require going through the gospel of John from the beginning up to chapter 14 and try and decide what commandments he gives. If you'll go ahead and do that, you'll find a very different Jesus with a very different focus than the one we've been looking at in Matthew.
Why Do I Think This Is Part of Scripture Mastery?
I think this verse was chosen to try and impress upon LDS youth the importance of keeping the commandments taught to them by Church leaders. Since they hope that their students love Jesus, this is Jesus telling them, from an LDS point of view, that this love should be expressed through such things as avoiding coffee, tea, and alcohol, keeping chaste, being honest with others, paying tithing, and preparing for missions and temple rituals. However, I'd argue that this interpretation relies upon approaching the New Testament as a coherent whole where all of the books within it are designed to complement each other. The truth is not so nice and simple, and if we read the statement from John within the context of the Gospel of John, the commandments in question seem to be related to loving Jesus and loving each other. I cannot get behind the first interpretation as it creates a situation ripe for abuse by Church leaders, both local and at the top, who might exercise "unrighteous dominion". The second interpretation is one that I whole-heartedly can get behind. The world needs more love between humans.