If God's sovereign redemptive plan is limited to the saving of individual souls then God's not as big as the Bible teaches, some could argue. Does God not have a plan for the entire cosmos? Does God not have a redemptive plan for his creation as well?
"Big God" theology is a theology that tells the story of a God who is sovereign over every aspect of the redemptive mission, which includes calling his people to himself and setting all things right in his creation so that the entire cosmos fulfill its intended purpose and design. This level of cosmic redemption is indeed a profound mystery because the Bible does not give us all the details in terms of what this looks like here and now. There is much that we do not know. However, we do know that God's ultimate work of redemption in Christ includes people, places, and things. God wants his people to care about the destinies of people and every aspect of his creation.
Henry Van Til, in the book The Calvinist Concept of Culture, observes:
The Calvinist does not become one-sidely Christological and soteriological in his interpretation of man's calling, but he continues to make the doctrines of creation and providence part of his working capital. He does not believe, as some other Christians seem to do, that God now excuses believers from their cultural calling due to the urgency of the missionary mandate, which calls the church to make disciples of all nations (223).
Abraham Kuyper, in Guidance for Christian Engagement In Government, observes,
[S]overeign authority flows out from God Almighty to all parts of his creation--to air and soil, to plant and animal, to a person's body and a person's soul, and in that soul to one's thinking, feeling, and will; and further, to a society in all its organic spheres of scholarship and business; and finally, to families, to rural and urban communities, and to the sphere that encompasses all these spheres and has to safeguard them all.
Thus political authority operates alongside many other authorities that are equally absolute and sacred in the natural and spiritual world, in society and family. Every attempt by political authority to try and rule over one the other areas is therefore a violation of God's ordinances, and resistance to is not a crime but a duty (italics his, 20).
A "Big God" theology that submits to God's sovereign, reign and rule, calling His people to himself and his reign over the entire creation is something, for example, that the tradition black church in America had to embrace in struggle for civil-rights for African Americans. It was God's "bigness" that gave the black church hope that not only would God set people free from the power of the devil but he would also intervene in the structural distortions in creation because he is sovereignly working in all spheres of life for His glory and the good of His creation. The Civil-Rights Movement was an example of the resistance duty Kuyper mentions as black church leaders, recognizing that the whole world belongs to God and God alone, said "no" to a political authority acting in ways contrary to God's design while conservative Protestants passively, and many actively, participated in the evils of American chattel slavery and Jim Crow.
The Westminster Confession of Faith describes God's sovereign providential rule this way:
God the great Creator of all things does uphold, direct, dispose, and govern all creatures, actions, and things, from the greatest even to the least, by His most wise and holy providence, according to His infallible foreknowledge, and the free and immutable counsel of His own will, to the praise of the glory of His wisdom, power, justice, goodness, and mercy.
A "Big God" theology that is limited to calling his people to himself without explication his plan to redeem everything in creation that was effected by the mutiny of Fall is not very big at all we might say. God's authority flows to every aspect of his creation, that's how big God is here and now and in the world to come (Col 1: 15-23; Dan 2:21; Heb 1:31, Dan 4:34-35, Ps 135:6; Acts 17; 25-26; Job 38-41; Matt 10:29; Prov 15;3; Acts 15:18; Ps 94:8-11, Eph 1:11; Ps 33:10-11; Isa 63:14, Eph 3:10; Rom 9:17; Ps 145:7).
Read Tim Keller's "Big God" theology of God's people and creation:
God has entered the world in Jesus Christ to achieve a salvation that we could not achieve for ourselves which now 1) converts and transforms individuals, forming them into a new humanity, and eventually 2) will renew the whole world and all creation. This is the ‘good news’—the gospel.
Now that's some real "Big God" theology right there. God's people and his creation. The revolution continues . . . .
Far As The Curse Is Found, Dr. Michael Williams (Covenant Theological Seminary)
Truth in All Its Glory: Commending the Reformed Faith, Dr. William Edgar (Westminster Theological Seminary)