Jonathan Edwards 
A Treatise Concerning Religious Affections [EPUB ebook] 

There is no question whatsoever, that is of greater importance to mankind, and that it more concerns every individual person to be well resolved in, than this:
What are the distinguishing qualifications of those that are in favor with God, and entitled to his eternal rewards? Or, which comes to the same thing,
What is the nature of true religion? And wherein do lie the distinguishing notes of that virtue and holiness that is acceptable in the sight of God? But though it be of such importance, and though we have clear and abundant light in the word of God to direct us in this matter, yet there is no one point, wherein professing Christians do more differ one from another. It would be endless to reckon up the variety of opinions in this point, that divide the Christian world; making manifest the truth of that declaration of our Savior, “Strait is the gate and narrow is the way, that leads to life, and few there be that find it.”
The consideration of these things has long engaged me to attend to this matter, with the utmost diligence and care, and exactness of search and inquiry, that I have been capable of. It is a subject on which my mind has been peculiarly intent, ever since I first entered on the study of divinity. But as to the success of my inquiries it must be left to the judgment of the reader of the following treatise.