Concerning the meaning of the phrase “sanctification has its ground in justification,” some out there in bloggo-land may be getting hung up on the term ground and reading more into to it than is warranted. Certainly there is no justification or sanctification to be found outside of our union with Christ. So that his gracious person and work are the ultimate ground and source of salvation, from election to glorification. Christ Jesus is the water we believers swim in and the water in which and through which God mercifully does all things. That being said, sanctification and justification are not parallel blessings independently given in Christ. Rather they are connected as effect is to cause as stated by this obscure reformer:
In short, I affirm, that not by our own merit but by faith alone, are both our persons and works justified; and that the justification of works [sanctification]depends on the justification [forensic] of the person , as the effect on the cause. (John Calvin, Acts of the Council of Trent with the Antidote)
Calvin further explains in Institutes 3.14.21:
21. Moreover, when Scripture intimates that the good works of believers are causes why the Lord does them good, we must still understand the meaning so as to hold unshaken what has previously been said–viz. that the efficient cause of our salvation is placed in the love of God the Father; the material cause in the obedience of the Son; the instrumental cause in the illumination of the Spirit, that is, in faith; and the final cause in the praise of the divine goodness. In this, however, there is nothing to prevent the Lord from embracing works as inferior causes. But how so? In this way: Those whom in mercy he has destined for the inheritance of eternal life, he, in his ordinary administration, introduces to the possession of it by means of good works. What precedes in the order of administration is called the cause of what follows. For this reason, he sometimes makes eternal life a consequent of works; not because it is to be ascribed to them, but because those whom he has elected he justifies, that he may at length glorify (Rom. 8:30); he makes the prior grace to be a kind of cause, because it is a kind of step to that which follows. But whenever the true cause is to be assigned, he enjoins us not to take refuge in works, but to keep our thoughts entirely fixed on the mercy of God; “The wages of sin is death; but the gift of God is eternal life,” (Rom. 6:23). Why, as he contrasts life with death, does he not also contrast righteousness with sin? Why, when setting down sin as the cause of death, does he not also set down righteousness as the cause of life? The antithesis which would otherwise be complete is somewhat marred by this variation; but the Apostle employed the comparison to express the fact, that death is due to the deserts of men, but that life was treasured up solely in the mercy of God. In short, by these expressions, the order rather than the cause is noted. The Lord adding grace to grace, takes occasion from a former to add a subsequent, so that he may omit no means of enriching his servants. Still, in following out his liberality, he would have us always look to free election as its source and beginning. For although he loves the gifts which he daily bestows upon us, inasmuch as they proceed from that fountain, still our duty is to hold fast by that gratuitous acceptance, which alone can support our souls; and so to connect the gifts of the Spirit, which he afterwards bestows, with their primary cause, as in no degree to detract from it.
Calvin uses the phrases free justification, the mercy of God, and gratuitous acceptance to point to the same thing: that the primary cause of everything pertaining to salvation flows from God’s sovereign grace and mercy as freely offered in the gospel of Jesus Christ. That which secures a man’s salvation is ultimately not dependent upon his works, i.e. his progressive sanctification. As both the Thirty-Nine Articles of Religion and the Westminster Confession of Faith teach, good works are the evidence of faith, the fruit of a true and lively faith, i.e. the effect. The reformers jealously guarded this understanding of salvation with the words by God’s grace alone, through faith alone, in Christ alone, lest the error of Rome would again find a foothold in the church. And it is this that the apostle Paul taught in order that – “… in the ages to come he might show the exceeding riches of his grace in kindness toward us in Christ Jesus: for by grace have ye been saved through faith; and that not of yourselves, it is the gift of God; not of works, that no man should glory.” (Eph. 2:7-9) ASV.