Horatius Bonar emphasizes the certainty of faith in his sermon 'Certainty of Certainties,' arguing that early Christians possessed a profound assurance of their relationship with God, rooted in the truth of Christ's coming and the promises of the Gospel. He contrasts this with the uncertainty many modern believers experience, highlighting that true knowledge of God leads to confidence, joy, and holiness. Bonar asserts that this certainty is not based on personal feelings or experiences but on the objective truth of the Gospel, which assures believers of their eternal life and identity as children of God. He encourages Christians to embrace this certainty, which should inspire humility, vigor, and a commitment to holy living. Ultimately, Bonar calls for a return to the apostolic confidence that characterized the early Church.