Because the covenant of works was broken by sin and was unable to confer life, God was pleased to proclaim the promise of Christ, the seed of the woman, as the means of calling the elect and producing in them faith and repentance (Ge 3:15). In this promise the gospel in its substance was revealed and made effectual for the conversion and salvation of sinners (Rev 13:8).
This promise of Christ and of salvation through him is revealed in the Word of God alone (Ro 1:17). The works of creation and providence, when assisted only by the light of nature, do not reveal Christ or grace through him, even in a general or obscure way (Ro 10:14-15, 17). Much less are those without the revelation of him in the promise or gospel enabled to attain saving faith or repentance by seeing these works of God (Pr 29:18; Isa 25:7; 60:2-3).
The gospel has been revealed to sinners in various times and in different places, along with the promises and precepts describing the obedience it requires. The particular nations and individuals who are granted this revelation are chosen solely according to the sovereign will and good pleasure of God (Ps 147:20; Ac 16:7). This choice does not depend on any promise to those who demonstrate good stewardship of their natural abilities based on common light received apart from the gospel. No one has ever done this nor can anyone do so (Ro 1:18-32). Therefore, in every age the preaching of the gospel to individuals and nations has been granted in widely varying degrees of expansion and contraction, according to the counsel of the will of God.
The gospel is the only outward means of revealing Christ and saving grace, and it is abundantly sufficient for that purpose. Yet to be born again, brought to life or regenerated, those who are dead in trespasses also must have an effectual, irresistible work of the Holy Spirit in every part of their souls to produce in them a new spiritual life (Ps 110:3; 1Co 2:14; Eph 1:19-20). Without this no other means will bring about their conversion to God (Jn 6:44; 2Co 4:4, 6).