Jeremiah 17

Jeremiah 17:1 the sin of Judah is written with a pen of iron
In our day, we would say, the sin is written in permanent ink.  You can’t erase their sin with an eraser.  They are past the point of forgiveness.  It won’t fade with time.  It is forever there before the Lord “written with a pen of iron.”  And what was the sin?  To break the first great commandment, “Thou shalt have no other gods before me” (Ex. 20:3).  This chapter shows God’s indignation that the Jews had broken commandments number 1 and 4, i.e. the command to have no other gods and the command to honor the Sabbath Day.
Jeremiah 17:2 their altars and their groves by the green trees upon the high hills
Jehovah is referring to their idolatrous altars hidden in secret groves.  He knew they had built places of idol worship up high in the hills among the green trees.  The sin was idolatry.
Jeremiah 17:4 I will cause thee to serve thine enemies in the land thou knowest not
The history of Israel is about to go full circle.  Remember how the whole story started?  They were servants in a foreign land, making bricks for Pharaoh, and living in the land of Goshen.  Now their rejection of the God who had brought them out of Egypt with a strong hand will land them in the exact same situation—in Babylon as slaves of Nebuchadnezzar (and later generations will live as servants in other heathen nations).  They had profaned the Promised Land.  That’s what the Lord means when he says, you shall “discontinue from thine heritage that I gave thee.”  He is saying, “I gave you the land of your inheritance; now I am going to take it away!”
Jeremiah 17:5 Cursed be the man that trusteth in man
Nephi said it best:  O Lord, I have trusted in thee, and I will trust in thee forever.  I will not put my trust in the arm of flesh; for I know that cursed is he that putteth his trust in the arm of flesh. Yea, cursed is he that putteth his trust in man or maketh flesh his arm. (2 Ne. 4:34)
“To have faith in Christ means to rely on his Spirit for guidance, reproof, comfort, and hope—and then to do what he tells us to do. If we rely on our own strength, intelligence, or goodness, we are vulnerable. Because of our mortal state, our abilities will sometimes fail us. And then we become diminished and feel worthless and eventually hopeless. But if we trust in God, rather than in the arm of flesh—even our own arm of flesh—he will never fail us.
“’O how great is the nothingness of … men’ (Hel. 12:7) is not a put-down but an accurate description of our power as weak mortals in comparison with God’s power. We must have faith in the Savior’s power to help us—and then truly allow him to change our hearts and purify them.” (Richard Chidester, “No Place for Pride,” Ensign, Mar. 1990, 19–20)
Neal A. Maxwell
We cannot safely trust "in the arm of flesh." Even when it is pumped full of steroids, it lacks the strength to maintain its grasp on the iron rod. Not alone does the arm of flesh finally prove anemic, but it always reaches for the wrong things (see 2 Chr. 32:8; Jer. 17:5). (Lord, Increase Our Faith, p. 87)
Jeremiah 17:8 a tree… that spreadeth out her roots by the river
"The magnificent sycamore fig tree grows in the river forests of Africa and parts of Asia Minor.  This giant of the forest dominates its surrounding terrain but is mainly know for its remarkable root structure. The roots are huge, flange-like buttresses that spread out at the base of the tree trunk and sink deeply into the earth. The roots enable the tree to withstand violent floodwaters of the rainy season, as well as to grow to great heights.
"The stability offered by its roots provides the sycamore fig with a natural advantage over other vegetation in the area…
"The Prophet Jeremiah understood this when he wrote:
   Blessed is the man that trusteth in the Lord, and whose hope the Lord is.
   For he shall be as a tree planted by the waters, and that spreadeth out her roots by the river, and shall not see when heat cometh, but her leaf shall be green; and shall not be careful in the year of drought, neither shall cease from yielding fruit. (Jer. 17:7-8.)
"…Just as the sycamore fig tree is buttressed by strong roots against destructive elements and grows tall among its surrounding vegetation, so the gospel and our faith in Jesus Christ fortifies us against the buffetings of Satan.
"As Latter-day Saints we should nourish the seeds of truth within us, tending carefully our spiritual growth, planting our roots deeply in truth, so that in times of stress, adversity, or temptation, we shall not wither or be cast down." (Plant Roots Deeply, LDS Church News, 1988, 07/09/88)
Jeremiah 17:10 I the Lord search the heart, I try the reins
Neal A. Maxwell
The Lord has spoken of temptations "to prove thee, to know what was in thine heart." (Deuteronomy 8:2.) If we link that scripture up with one in Jeremiah in which the Lord says, "I the Lord search the heart, I try the reins, even to give every man according to his ways" (Jeremiah 11:20, 17:10), we see the ultimate expression of agency and divine justice. If each of us really finally receives that which has been really wanted, none could quarrel with the justice of God. Trials and tribulations tend to squeeze the artificiality out of us, leaving the essence of what we really are and clarifying what we really yearn for. Therefore, the record will be clear.
Paul said that "we must through much tribulation enter into the kingdom of God." (Acts 14:22.) "Every man's work shall be made manifest: for the day shall declare it, because it shall be revealed by fire; and the fire shall try every man's work of what sort it is." (1 Corinthians 3:13.)
Joseph Smith
God hath said that He would have a tried people, that He would purge them as gold, now we think that this time He has chosen His own crucible, wherein we have been tried; and we think if we get through with any degree of safety, and shall have kept the faith, that it will be a sign to this generation, altogether sufficient to leave them without excuse; and we think also, it will be a trial of our faith equal to that of Abraham, and that the ancients will not have whereof to boast over us in the day of judgment, as being called to pass through heavier afflictions; that we may hold an even weight in the balance with them. (Teachings of the Prophet Joseph Smith, 135-136
Jeremiah 17:12-18 The Psalm of Jeremiah
Much has been said of the psalm of Nephi (2 Ne. 4:15-35).  Well, this passage has that psalm-like quality to it as well.  In these passages we see into the heart of great hearts, hearts the Lord is stretching to their breaking point.  How much can Jeremiah take?  That is the question.
   A glorious high throne from the beginning is the place of our sanctuary. (We started this process with our Heavenly Father in the pre-mortal world)
   O LORD, the hope of Israel, all that forsake thee shall be ashamed, and they that depart from [thee] shall be written in the earth, because they have forsaken the LORD, the fountain of living waters.
   Heal me, O LORD, and I shall be healed; save me, and I shall be saved: for thou art my praise.
   Behold, they say unto me, Where is the word of the LORD? let it come now.
   As for me, I have not hastened from being a pastor to follow thee: neither have I desired the woeful day; thou knowest: that which came out of my lips was right before thee.
   Be not a terror unto me: thou art my hope in the day of evil.
   Let them be confounded that persecute me, but let not me be confounded: let them be dismayed, but let not me be dismayed: bring upon them the day of evil, and destroy them with double destruction.
Chronologically, it seems that this chapter fits into Jeremiah’s later years when the persecutions became more severe.  At times he was put in stocks, at others lowered into a miry dungeon, still others the King sought his life.  He was very alone and had to rely on the Lord for strength.  How great is the cry of his soul, “Heal me, O Lord, and I shall be healed; save me, and I shall be saved: for thou art my praise”?
Jeremiah 17:21 Thus saith the Lord; Take heed to yourselves, and bear no burden on the sabbath day
James E. Talmage
The observance of the Sabbath as a holy day was prominent among the Lord's requirements of His people, Israel, from a very early period in their history as a nation. Indeed, the keeping of the Sabbath as a day of surcease from ordinary toil was a national characteristic, by which the Israelites were distinguished from pagan peoples, and rightly so, for the holiness of the Sabbath was made a mark of the covenant between the chosen people and their God. (Jesus the Christ, 190)
Jeremiah 17:24 If ye diligently hearken unto me, saith the Lord, to… hallow the Sabbath day
We know that there is a blessing for keeping every commandment the Lord gives us (D&C 130:20-21), but often the specific blessing is not listed.  In this passage, the Lord tells the Jews what is at stake.  He tells them the blessing they can expect for keeping the Law of the Sabbath.  Perhaps the blessing is not what we would have expected:
Then shall there enter into the gates of this city kings and princes sitting upon the throne of David, riding in chariots and on horses, they and their princes, the men of Judah, and the inhabitants of Jerusalem: and this city shall remain for ever.
That is a huge blessing for keeping the Sabbath day!  Especially when you compare it to the alternative: the king is dethroned, the gates of the city burned, and the palaces of Jerusalem plundered.  Do we consider how great the blessings are for Sabbath observance? Isaiah said if we keep the Sabbath, Then shalt thou delight thyself in the Lord; and I will case thee to ride upon the high places of the earth, and feed thee with the heritage of Jacob thy father: for the mouth of the Lord hath spoken it (Isa. 58:14).
Bruce R. McConkie
The Sabbath is the leaven of life. Our conduct on all days is influenced by what we do on the Lord's day. Ancient Israel received this command: "Bear no burden on the sabbath day. . . . Hallow ye the sabbath day." That is: 'Rest from your labors, and worship the Lord.' Had they done so, the Lord would have preserved their kingdom forever. (Jeremiah 17:20-27.) Similarly, if we would keep the Sabbath, the Lord would bless and prosper us beyond anything we have ever known.
The Sabbath is the day appointed on which we should chart a course to eternal life. On it we partake of the sacrament and make the covenant that enables us to have the Lord's Spirit with us on all days and at all times. (A New Witness for the Articles of Faith [Salt Lake City: Deseret Book Co., 1985], 301)