This will undoubtedly be beneficial to any that endeavor to preach. But, perhaps, this will be even more helpful for my Calvinist brothers that struggle with knowing how to preach the doctrines of grace consistently.
We should undoubtedly endeavor to maintain a consistency in our preaching; but unless we keep the plan and manner of the Scripture constantly in view, and attend to every part of it, a design of consistency may fetter our sentiments, and greatly preclude our usefulness. We need not wish to be more consistent than the inspired writers, nor be afraid of speaking as they have spoken before us. We may easily perplex ourselves and our hearers, by acute reasoning on the nature of human liberty, and the Divine agency on the hearts of men; but such disquisition's are better avoided. We shall, perhaps, never have full satisfaction on these subjects, until we arrive in the world of light. In the mean time, the path of duty, the good old way, lies plainly before us. (emphasis mine)
You can read the entire letter here.
Just in case you are tripped up by some of the older language that Newton uses allow me to summarize. In this letter Newton is addressing a young man that seems to be influenced by some Hyper-Calvinistic teachings. The young man wonders in what way he can legitimately offer free grace to unbelievers that may not be God’s will to regenerate.
Newton in this address is telling the young man that you must preach the gospel to all—that is the example the Word lays out to us. In this particular paragraph Newton is simply saying do not worry about being “consistent” in places where the inspired writers are not. Paul can hold “free grace” highly at the same time he calls “all men everywhere to repent”. Newton is simply saying, do the same thing.
(FYI, I am pretty confident that is a picture of Whitefield and not Newton).