The Church teaches that Jesus is Jehovah, the God of the Old Testament. But I have a hard time understanding what this means. It seems to me, for example, that Old Testament peoples worshipped the God of the Old Testament. Seems pretty obvious, except for what Jacob has to say about it:
For, for this intent have we written these things, that they may know that we knew of Christ, and we had a hope of his glory many hundred years before his coming; and not only we ourselves had a hope of his glory, but also all the holy prophets which were before us. Behold, they believed in Christ and worshiped the Father in his name, and also we worship the Father in his name. (Jacob 4:4-5)
It seems to me also that Old Testament peoples prayed to the God of the Old Testament -- again, seems like a fairly obvious claim, except for what Nephi says about it: “Ye shall pray unto the Father in the name of Christ” (2 Nephi 32:9). (Leaving aside for the moment the fact that there are plenty of Old Testament prayers addressed to Jehovah.)
It seems to me also that if God is “the same yesterday, today, and forever”, then the God of the Old Testament would also be the God of the New Testament and the God of the Doctrine & Covenants. It seems to me that this would be the God we worship and pray to, except that we have also been taught to pray to the Father, rather than to Christ.
Call me confused, but it seems a little odd to me to say that someone we are not supposed to pray to or worship is God, or that someone the Old Testament peoples didn't worship or pray to was their God.
The only resolution I can come to is the idea that there are multiple Gods who have different roles. Margaret Barker's Old Testament studies suggest that pre-exilic Hebrews had such an understanding, closer to what Nephi teaches about the Father and the Son. But if this is correct, and the Hebrews recognized more than one God (and we do, too), then I'm not sure what we mean by our claim that Jesus is the God of the Old Testament.