Many readers will be interested to learn that Dr. Lloyd-Jones preached evangelistic sermons from the Old Testament. This was as unusual in his day as were his doctrinal sermons. Murray notes that ML-J is often thought of today as a teaching preacher, but his wife, Bethan Lloyd-Jones, considered her husband to be first and foremost an evangelist (55).
This contemporary misapprehension may simply be due to the sermons that have been published as over against those which have not been released to the public in print. The assumption of publishers was that Christians would make up the lion's share of his readers, so the sermons that went to press were ones that it was thought were more geared to believers already in the way. But Murray points out that in reality, "more than half" of Lloyd-Jones' preaching was evangelistic. As already noted, the "Doctor" often used Old Testament texts in his evangelistic sermons and this was because he saw the "neglect and near disappearance of the Old Testament as exercising a detrimental influence on contemporary Christianity" (61).
Unfortunately it seems the problem is still with us these many years later. ML-J also thought that the disuse of the OT would have serious practical consequences. For these reasons ML-J used the Old Testament for evangelistic purposes. The benefit of using the OT in this context involved the fact that as Scripture, the OT was effective in revealing sin in all its true colors. It also pointed to the fact that a life lived without God was futile. Ultimately Lloyd-Jones preached evangelistically from the OT because it was a "book about God" (76).
From: Lloyd-Jones: Messenger of Grace By Iain Murray