How so?

(Thomas Brooks, "Paradise Opened" 1675)

All mankind would have been eternally lost, had
God not, of His own free grace and mercy, made
a covenant of grace with poor sinners.

"They will be My people, and I will be their God."
     Jeremiah 32:38

This is a comprehensive promise, for God to be
our God—it includes all.

The covenant of grace is an agreement, which God has
made with sinful man, out of His mere mercy and grace,
wherein He undertakes for fallen man, to make him
everlastingly happy. God engages that He will be our
God; that is, as if He said, "You shall have an interest
in all My attributes for your good:
  My grace shall be yours to pardon you,
  My power shall be yours to protect you,
  My wisdom shall be yours to direct you,
  My goodness shall be yours to relieve you,
  My mercy shall be yours to supply you,
  My glory shall be yours to crown you."

"I will make an everlasting covenant with them;
 that I will not turn away from them, to do them
 good; and I will put My fear into their hearts—
 that they shall not depart from Me." Jer. 32:40

The covenant of grace is everlasting on God's part,
and also on our part. On God's part, "I will never
turn away from them to do them good." And on our
part, "they shall never depart from Me." How so?
"I will put My fear into their hearts—that they shall
not depart from Me." That they shall persevere, and
hold out to the end—I will so deeply rivet a reverent
dread of Myself in their souls—as shall cause them
to believe, love, repent, obey, cling and cleave, and
keep close to Me forever.

O sirs! this is the glory of the covenant of grace—
that whatever God requires on man's part, that He
undertakes to perform for man!

"I will also sprinkle clean water on you, and you will
 be clean. I will cleanse you from all your impurities
 and all your idols. I will give you a new heart and
 put a new spirit within you; I will remove your heart
 of stone and give you a heart of flesh. I will place My
 Spirit within you and cause you to follow My statutes
 and carefully observe My ordinances." Ezek. 36:25-27