How can diluted truth be defined? It is when the truths are mixed with error which confuses people which can deceive just anybody. There are many teachings today where truth is subtly mixed with dangerous error. Many of them get the Bible and hold it but they insert their own deadly doctrine into it. Now no theologian is perfect and they are bound to error but when it comes to the truth of the Gospel according to Jesus, it cannot afford to be diluted. Many times today, we have preachers who add something else to the Gospel. Many of these errors range from the error of conditional security or the varying degrees of "easy Christianity" or which some call "easy believism". One can add works to a condition to getting and/or staying saved or subtracting works from the Christian life as a result of being saved. Both of them are examples of truth diluted with error.
One can always say, "Well he is a man of God because he quoted Scriptures." Excuse me but did you know Satan quoted Scriptures when he tempted Jesus in the wilderness? Just because a person is quoting Scriptures does not mean the person is not misquoting Scriptures. One can go ahead and quote Ephesians 2:8-9 to justify Antinomianism, Matthew 7:21-23 to justify conditional security, Matthew 16:16-18 to justify the unbiblical office of the papacy, 1 Corinthians 7:32-35 to justify priestly celibacy, Isaiah 24:15 to justify the existence of the Iglesia ni Cristo (Manalo) as the "true church" and the list goes on and on. These are dangerous truths which I remembered how I once thought certain "Christian" denominations were Christian until I investigated their official statements with the Bible. Some of them even misquoted several passages to "prove" Jesus was not God on Earth or that faith is not enough to save without taking things into context. Anybody can go ahead and quote Scriptures but is misquoting them in the process.
To fight diluted truth it can only be if you really engage in proper exegesis. If you really read the Bible cover to cover, get the keywords, study the meaning in context then you are bound to have less error. No Bible teacher is perfect but certainly we can avoid error that destroys the soul. I could care less about the issue of a futurist vs. historicist view of Revelation (and some have gone as far as to say futurism is a Jesuit heresy) and Charles H. Spurgeon still had some mistakes in his theology but they were not soul-damning. Even a Calvinist and non-Calvinist can both agree that true converts are destined to endure to the end, that true faith begets good works and that to teach salvation is a license to sin is just as deadly as to teach salvation by works.