The Sermon on the Mount was the beginning of our Lord's ministry in preparing His disciples. Since the Holy Spirit is the Author of the bible, we must let scriptures explain scriptures and you will be amazed how His words, parables, teachings, etc. reveal His truth.
Parable of Laying Treasures
Mat 6:19-23 "Lay not up for yourselves treasures upon earth, where moth and rust doth corrupt, and where thieves break through and steal: But lay up for yourselves treasures in heaven, where neither moth nor rust doth corrupt, and where thieves do not break through nor steal: For where your treasure is, there will your heart be also. The light of the body is the eye: if therefore your eye be single, your whole body shall be full of light. But if your eye be evil, your whole body shall be full of darkness. If therefore the light that is in you be darkness, how great is that darkness!"
So when we set our hearts to please Him, we will keep His words, will and commandments.
1 John 3:22 And whatever we ask we receive from Him, because we keep His commandments and do those things that are pleasing in His sight.
Therefore, if your eye be single – staying focus to pleasing Him alone; then we will be able to understand His will and the riches of His eternal kingdom, and we will be full of the fruits of the Holy Spirit, and the joy when doing all types of righteous and merciful works.
Mat 19:21 Jesus said to him, “If you want to be perfect, go, sell what you have and give to the poor, and you will have treasure in heaven, and come, follow Me.” (Read also Mark 10:21, Luke 12:33; 18:22)
But if your eye is evil – love of this world and the things therein, selfish, covetous; then we will be in complete darkness - without any understanding of His words and will.
1 Tim 6:10 For the love of money is a root of all kinds of evil, for which some have strayed from the faith in their greediness, and pierced themselves through with many sorrows.
'If therefore the light that is in you be darkness, how great is that darkness!' When the Holy Spirit (light becomes darkness) leaves a Christian, he has lost his salvation, and will display bad fruits -anger, murmurs, divisive, lies, deceptions, merciless, sinful behaviours, etc. His wicked schemes to deceive and cheat other Christians (great darkness) will be worse than those of the heathens. These will be the ‘wolves in sheepskin’ in churches.
2 Pet 2:20-21 For if, after they have escaped the pollutions of the world through the knowledge of the Lord and Savior Jesus Christ, they are again entangled in them and overcome, the latter end is worse for them than the beginning. For it would have been better for them not to have known the way of righteousness, than having known it, to turn from the holy commandment delivered to them.
Parable of the Beautiful Pearl
Matthew 13:45-46 “Again, the kingdom of heaven is like a merchant seeking beautiful pearls, who, when he had found one pearl of great price, went and sold all that he had and bought it.”
This merchant found the beautiful pearl which is entrance to the Kingdom of God and immediately he sold all he had and gave it to the poor so that he will be able to enter the Kingdom of God.
Rev 21:21-22 The twelve gates were twelve pearls: each individual gate was of one pearl. And the street of the city was pure gold, like transparent glass. But I saw no temple in it, for the Lord God Almighty and the Lamb are its temple.
Luke 18:22-25 So when Jesus heard these things, He said to him, "You still lack one thing. Sell all that you have and distribute to the poor, and you will have treasure in heaven; and come, follow Me.” But when he heard this, he became very sorrowful, for he was very rich. And when Jesus saw that he became very sorrowful, He said, "How hard it is for those who have riches to enter the kingdom of God! For it is easier for a camel to go through the eye of a needle than for a rich man to enter the kingdom of God."