He that loveth not, knoweth not God - Has no true acquaintance with God; has no just views of him, and no right feelings toward him. The reason for this is implied in what is immediately stated, that "God is love," and of course if they have no love reigning in their hearts, they cannot pretend to be like him.
For God is love - He is not merely benevolent, he is benevolence itself. Compare the notes at 2-Corinthians 13:11. Never was a more important declaration made than this; never was more meaning crowded into a few words than in this short sentence - "God is love." In the darkness of this world of sin - in all the sorrows that come now upon the race, and that will come upon the wicked hereafter - we have the assurance that a God of infinite benevolence rules over all; and though we may not be able to reconcile all that occurs with this declaration, or see how the things which he has permitted to take place are consistent with it, yet in the exercise of faith on his own declarations we may find consolation in "believing" that it is so, and may look forward to a period when all his universe shall see it to be so. In the midst of all that occurs on the earth of sadness, sin, and sorrow, there are abundant evidences that God is love.
In the original structure of things before sin entered, when all was pronounced "good;" in the things designed to promote happiness, where the only thing contemplated is happiness, and where it would have been as easy to have caused pain; in the preservation of a guilty race, and in granting that race the opportunity of another trial; in the ceaseless provision which God is making in his providence for the wants of unnumbered millions of his creatures; in the arrangements made to alleviate sorrow, and to put an end to it; in the gift of a Saviour more than all, and in the offer of eternal life on terms simple and easy to be complied with - in all these things, which are the mere expressions of love, not one of which would have been found under the government of a malignant being, we see illustrations of the sublime and glorious sentiment before us, that "God is love." Even in this world of confusion, disorder, and darkness, we have evidence sufficient to prove that he is benevolent, but the full glory and meaning of that truth will be seen only in heaven. Meantime, let us hold on to the truth that he is love. Let us believe that he sincerely desires our good, and that what seems dark to us may be designed for our welfare; and amidst all the sorrows and disappointments of the present life, let us feel that our interests and our destiny are in the hands of the God of love.

He that loveth not - As already described, knoweth not God - has no experimental knowledge of him.
God is love - An infinite fountain of benevolence and beneficence to every human being. He hates no thing that he has made. He cannot hate, because he is love. He causes his sun to rise on the evil and the good, and sends his rain on the just and the unjust. He has made no human being for perdition, nor ever rendered it impossible, by any necessitating decree, for any fallen soul to find mercy. He has given the fullest proof of his love to the whole human race by the incarnation of his Son, who tasted death for every man. How can a decree of absolute, unconditional reprobation, of the greater part or any part of the human race, stand in the presence of such a text as this? It has been well observed that, although God is holy, just, righteous, etc., he is never called holiness, justice, etc., in the abstract, as he is here called Love. This seems to be the essence of the Divine nature, and all other attributes to be only modifications of this.

He that loveth not knoweth not God; (8) for God is love.
(8) A confirmation: for it is the nature of God to love men, of which we have a most manifest proof above all other, in that of his only free and infinite good will towards us his enemies, he delivered to death, not a common man, but his own Son, indeed his only begotten Son, to the end that we being reconciled through his blood might be partakers in his everlasting glory.

He that loveth not, knoweth not God,.... If a man loves not the children of God, those that are born of him, he does not know, so as to love God, the Father of them; for to pretend love to God, the begetter of them, whom he sees not, and not love those who are begotten by him, and are visible objects of respect, is a contradiction, and cannot be reconciled: see 1-John 4:20. This clause is left out in the Ethiopic version, and is transposed in the Syriac version, which reads the text thus, "for God, is love, and whoever loveth not, knoweth not God". By which reading, the following reason stands in close connection with 1-John 4:7.
For God is love; he loves himself; there is an entire love between the three divine Persons, who are in the strictest, and in the most inconceivable and inexpressible manner affected to each other; their love is natural and essential: God loves all his creatures as such, nor does he hate any of them, as so considered; and he bears an everlasting, unchangeable, and invariable love to his elect in Christ Jesus; of which an instance is given in the following verses, and is a reason why the saints should love one another; that they might be like their heavenly Father, by whom they are begotten, and of whom they are born, and whose children they are; seeing he is love itself, and in his breast is nothing else but love. So the Shekinah is, by the Cabalistic Jews (t), called "love".
(t) Shirhashirim Rabba, fol. 15. 1. & Lex. Cabal. p. 43, 44.

knoweth not--Greek aorist: not only knoweth not now, but never knew, has not once for all known God.
God is love--There is no Greek article to love, but to God; therefore we cannot translate, Love is God. God is fundamentally and essentially LOVE: not merely is loving, for then John's argument would not stand; for the conclusion from the premises then would be this, This man is not loving: God is loving; therefore he knoweth not God IN SO FAR AS GOD IS LOVING; still he might know Him in His other attributes. But when we take love as God's essence, the argument is sound: This man doth not love, and therefore knows not love: God is essentially love, therefore he knows not God.

God is love - This little sentence brought St. John more sweetness, even in the time he was writing it, than the whole world can bring. God is often styled holy, righteous, wise; but not holiness, righteousness, or wisdom in the abstract, as he is said to be love; intimating that this is his darling, his reigning attribute, the attribute that sheds an amiable glory on all his other perfections.