A Short Treatise on the Love of God

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possesses Love, then in him the Law and the Prophets have served their purpose.
The "Law and the Prophets" is a common biblical shorthand for the entire Hebrew Scriptures (the Old Testament). The author suggests that love is the ultimate goal of all scripture.

3. But this single and necessary Commandment of Love is completely and entirely forgotten, or is judged as unnecessary throughout the whole world; people never become possessors of Love; yet they still wish to be called Christians. Is Jesus Christ then not the Love of the Father? How then does their behavior resemble Him? Must Man not also bear once again the Image and Likeness of God?
The "Image and Likeness of God" refers to the concept of Imago Dei from the Book of Genesis, suggesting that humans are meant to reflect God's character.
If so, where then is the Love of Jesus? You think, and you shall all say, that you have it; but it is far otherwise, as you shall be able to see with conviction in the following pages—provided you still have any glimmer of natural understanding left.
"Natural understanding" refers to the innate human ability to reason and perceive truth, which the author fears has been clouded by worldly living.

4. Verily, verily, the Lord God even now requires no more of you all than to fulfill this Commandment alone: it is the Most Ancient Commandment that Man has had from the beginning, and it is also the only one that shall remain for eternity. And now it is given to you all once again as a new Commandment, because none of you follow it. Therefore, know this as a firm truth: he who does not have Love, has nothing of God.