What is the The Greatest Commandment?

In the OT, we have the 10 commandments. They are summarised as "love God and love others as oneself" (Mat 22:37-40).

In the NT, we come across other commandments. Below is a list:

Mat 6:33 "But seek first His kingdom and His righteousness, and all these things will be added to you.

Mat 28:19-20 The Great Commission
(19) "Go therefore and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit, (20) teaching them to observe all that I commanded you; and lo, I am with you always, even to the end of the age."

1Jo 3:23 This is His commandment, that we believe in the name of His Son Jesus Christ, and love one another, just as He commanded us. (Joh 13:34)

Joh 13:34-35 NASB
(34) "A new commandment I give to you, that you love one another, even as I have loved you, that you also love one another. (35) "By this all men will know that you are My disciples if you have love for one another."

Mat 22:37-40 (37) And He said to him, " 'YOU SHALL LOVE THE LORD YOUR GOD WITH ALL YOUR HEART, AND WITH ALL YOUR SOUL, AND WITH ALL YOUR MIND.' (38) "This is the great and foremost commandment. (39) "The second is like it, 'YOU SHALL LOVE YOUR NEIGHBOR AS YOURSELF.' (40) "On these two commandments depend the whole Law and the Prophets."

The "LOVE GOD" part in the OT is now "Believe in His Son Jesus Christ" of 1 Jo 3:23 and then love one another with His love.

So which one is the greatest of all? How are they related?

Luk 10:40-42 (40) But Martha was distracted with all her preparations; and she came up to Him and said, "Lord, do You not care that my sister has left me to do all the serving alone? Then tell her to help me." (41) But the Lord answered and said to her, "Martha, Martha, you are worried and bothered about so many things; (42) but only one thing is necessary, for Mary has chosen the good part, which shall not be taken away from her."

The Only One Thing is to be at Jesus's feet. i.e. to be with Jesus all the time as His disciple. To know and to have an intimate relationship with Him.

There are other Supporting Verses:
If you go through Paul's letters and his prayers for the Believers, you will find that he often prayed that they know Jesus more. Phil 3:8 Knowing Jesus is the most valuable thing of all.

Eph 1:16-19 (16) ... making mention of you in my prayers; (17) that the God of our Lord Jesus Christ, the Father of glory, may give to you a spirit of wisdom and of revelation in the knowledge of Him. (18) I pray that the eyes of your heart may be enlightened so that you will know what is the hope of His calling, what are the riches of the glory of His inheritance in the saints, (19) and what is the surpassing greatness of His power toward us who believe. These are in accordance with the working of the strength of His might.

Eph 3:18... to comprehend with all the saints what is the breadth and length and height and depth, 19 and to know the love of Christ that surpasses knowledge, that you may be filled with all the fullness of God. 

Col 1:9 For this reason also, since the day we heard of it, we have not ceased to pray for you and to ask that you may be filled with the knowledge of His will in all spiritual wisdom and understanding,

Summary

To know Jesus Christ intimately is the basic and greatest commandment.

1. The knowing is not just mental but with the heart and with experience through following His will for us. 
2. It is to know of His love for us that we may trust and obey His will for us. 
3. Being rooted in His love, then we could love others as He loves them. 
4. We can then execute the Great Commission to go to share the gospels and mentor them to be like Christ too.

John 13:35  By this all men will know that you are my disciples if you love one another.

In conclusion, the greatest commandment is to love God by knowing and believing in Jesus, which leads to loving others and fulfilling the Great Commission. This relationship with Jesus is the foundation of all Christian living.