Four points from John Flavel:
First, That Christ and his benefits go inseparably and undividedly together: it is Christ himself who is made all this unto us: we can have no saving benefit separate and apart from the person of Christ: many would willingly receive his privileges, who will not receive his person; but it cannot be; if we will have one, we must take the other too: Yea, we must accept his person first, and then his benefits: as it is in the marriage covenant, so it is here.
Secondly, That Christ with his benefits must be personally and particularly applied to us before we can receive any actual, saving privilege by him; he must be [made unto us] i.e. particularly applied to us; as a sum of money becomes, or is made the ransom and liberty of a captive, when it is not only promised, but paid down in his name, and legally applied for that use and end. When Christ died, the ransom was prepared, the sum laid down; but yet the elect continue in sin and misery, notwithstanding, till by effectual calling it be actually applied to their persons, and then they be made free, Rom. v.10,11 reconciled by Christ's death, by whom "we have not received the atonement."
Thirdly, That this application of Christ is the work of God, not of man: "Of God he is made unto us:" The same hand that prepared it, must also apply it, or else we perish, notwithstanding all that the Father hath done in contriving, and appointing, and all that the Son hath done in executing, and accomplishing the design thus far. And this actual application is the work of the Spirit, by a singular appropriation.
Fourthly, and lastly, This expression imports the suitableness of Christ, to the necessities of sinners; what they want, he is made to them; and indeed, as money answers all things, and is convertible into meat, drink, raiment, physic, or what else our bodily necessities do require; so Christ is virtually, and eminently all that the necessities of our souls require; bread to the hungry, and clothing to the naked soul. In a word, God prepared and furnished him on purpose to answer all our wants, which fully suits the apostle's sense, when he saith, "Who of God is made unto us wisdom and righteousness, sanctification and redemption."
John Flavel, works, Vol. 1 pp.17-18.