"No man can serve two masters; for either he will hate the one and love the other, or else he will hold to the one and despise the other. Ye cannot serve God and Mammon."
This is Christ's great division. We all have but one Master. Choose carefully.
This is another way to describe the "jealous God" of the Old Covenant. (Ex. 34: 14.) Since He requires everything of you, any holding back is infidelity to Him.
Christ is reaffirming God's primacy. Your affection for Him cannot be shared. It simply can't be done. When attempted, it shows you "hate" or "despise" Him, because when you share your fidelity, you reject His direction.
It is this principle that justified the earlier prophets in likening Israel to a "harlot" or a "whore" when she worshiped other gods. (See, eg. Jeremiah 3: 6; Hosea 9: 1; Judges 8: 33.) It is not possible to be converted to the Lord and not be devoted to Him.
All of what is "Mammon" is subordinate to God. The Lord's ways require the things you have in this life to be used for His purposes and according to His desire. His commandments cover all things, and you cannot divorce your temporal concerns from His teachings. (D&C 29: 35.)
Devotion to Him requires that what you do, say, and think be aligned with Him. Conversion is a progressive process where you develop to be more like Him throughout life. You can't just "get a testimony" and then not be completely converted to Him. He expects to completely remake you. This sermon is the blueprint for the new creation you are to become.
This statement deals in absolutes because the Lord's way is the way of absolutes. He can accept nothing less than all. The adversary knows this and is content with getting even a little from you. The adversary knows that a little compromise is everything when compromising your faithfulness to the Lord.
The world will accept anything half-hearted. The world knows you love it, if you will just give in a little to its persuasion. Contamination is contamination and will eventually poison you. So any degree of unrighteousness is enough to please the world. For the Lord, however, it is all or nothing. It is complete fidelity to Him which alone will satisfy. Keeping one foot in the world, while giving lip service to Him will never meet the requirements for loving Him. (D&C 1: 31.)
Those who think the Lord is announcing a new, easier system to replace the earlier, more demanding Law of Moses do not understand His teachings. This is far more exact and moves the battleground into your heart. He is asking you to transform the soul. He is asking you to become like Him. This is not outward observances. However troubling and wearisome those may have been, they were at least something that could be done without battling in your heart with motive, intent and desires. Here Christ wants you to conform everything, even your desires, to be instruments of your salvation.
This is a call to a much higher way of life. It is a much deeper and more meaningful way to approach God. It is inside you.