The "wherefore" at the beginning of this verse returns to the theme of the mercies of God. It is because we know Him to be merciful that we are willing to approach to beg that mercy. Of course the mercy of God comes only through Jesus Christ, and therefore Jacob exhorts us to be reconciled to the Father through the Son. This faith in Jesus is effective even before the actions that effect the atonement. It is the faithful promise of God and Jesus that the atonement will be made that allows it to be effective even before it is accomplished. This is, of course, absolutely essential, for there is no repentance without the atonement, yet there were all of the children of God who died before the atonement who were still in need of the cleansing power of repentance in their lives. It is this principle that Jacob is applying here. The faithful Nephites may obtain reconciliation to God through Jesus, eve though the atonement itself had not yet occurred.
Literary: The language Jacob employs is very similar to Paul: "Rom. 5:2 By whom also we have access by faith into this grace wherein we stand, and rejoice in hope of the glory of God."
While Jacob's meaning is certainly the same, the similarity in the wording is no doubt influenced by the KJV passage in Romans. That is not to say that Joseph Smith copied the phrase, but rather that the familiarity of the terms came from Joseph's understanding of Paul.