In his article "Faith" in Vol 3 of his collected writings, Warfield reminds us of the centrality of faith in the lives of the patriarchs.
The patriarchal religion is essentially a religion, not of law but of promise, and therefore not primarily of obedience but of trust; the holy walk is characteristics of God's servants, but it is characteristically described as a walk 'with God'; its peculiarity consisted precisely in the ordering of life by entire trust in God, and it expressed itself in conduct growing out of this trust. The righteousness of the patriarchal age was thus but the manifestation in life of an entire self-commitment to God, in unwavering trust in His promises.
--B. B. Warfield, "Faith," in Biblical and Theological Studies (ed. S. G. Craig; Philadelphia: P&R, 1952), 406-7
Some may be interested in hearing D. A. Carson's striking comments on Warfield in this June 2008 interview with Mark Dever.