And thou shalt know the Lord - This knowledge of God follows on God's act of betrothal and of love. "We love God, because God first loved us." And the true knowledge of God includes the love of God. "To love man, we must know him: to know God, we must love Him." To "acknowledge" God, is not yet to "know" Him. They who love not God, will not even acknowledge Him as He Is, "Supreme Wisdom and Goodness and Power, the Creator and Preserver; the Author of all which is good, the Governor of the world, Redeemer of man, the most bounteous Rewarder of those who serve Him, the most just retributor of those who persevere in rebellion against Him." They who will not love God, cannot even "know" aright of God. But to "know God," is something beyond this. It is to know by experience that God is good; and this God makes known to the soul which he loves, while it meditates on Him, reads of Him; speaks of Him, adores Him, obeys Him. "This knowledge cometh from the revelation of God the Father, and in it is true bliss. Whence, when Peter confessed Him to be the Son of man and Son of God, He said, "Blessed art thou, for Flesh and blood hath not revealed it unto thee, but My Father which is in heaven." Yea, this knowledge is life eternal, as He said, "This is life eternal, that they might know Thee the only true God, and Jesus Christ whom Thou hast sent" John 17:3.

In faithfulness - Thou shalt no more prostitute thyself to idols, but be faithful to him who calls himself thy husband.
Thou shalt know the Lord - There shall be no more infidelity on thy part nor divorce on mine; and thou shalt experience me to be the sole, present, and eternal good of thy immortal spirit: and when this conviction is fully rooted then there can be no more idolatry, for it shall be seen that an idol is nothing in the world.

I will even betroth thee unto me in (y) faithfulness: and thou shalt know the LORD.
(y) With a covenant that will never be broken.

I will even betroth thee unto me in faithfulness,.... Which lies in keeping the marriage contract inviolable; Christ will never suffer his faithfulness to fail, nor break his covenant; as he is faithful to his Father that appointed him, so he is, and will be, to his church and people, and to every believer, to whom he is espoused; and it is he that makes them faithful unto him, and gives them faith to believe in him, receive, embrace, own, and acknowledge him as their husband: and in this sense some understand it, rendering it, "in faith" (z); so the Targum and others. This is the third time the word "betroth" is used, or this promise made; which, according to Jerome, refers to them espousing of the Jews in Abraham, at Mount Sinai, and in the times of Christ; and, according to Kimchi, to the three captivities of the Jews, in Egypt, in Babylon, and that in which they now are: and some Christian writers think the mystery of the Trinity is here pointed at; and the sense to be, that all the three divine Persons, Father, Son, and Spirit, would espouse them: but rather it is so often repeated to confirm it, and express the certainty of it, which might, on many accounts, seem a thing incredible.
And thou shall know the Lord; that the Messiah is Jehovah, and that he is their husband; they shall all know him, from the least to the greatest; they shall have a saving knowledge of him, which will issue in eternal life; they shall own him, and acknowledge him, serve and obey him, as their Lord, Head, and Husband, as well as love him, and believe in him. The Targum is,
"and ye shall know to fear before the Lord;''
see Jeremiah 31:34. Let it be observed, here are no conditions throughout, it is only "I will", and "thou shalt".
(z) "in fide", V. L. &c.

faithfulness--to My new covenant of grace with thee (1-Thessalonians 5:24; Hebrews 10:23).