The greatest commandment according to Jesus is found in the book of Matthew 22:37 and it is to love God with all that you are. It is mentioned more than once in the Bible.

• “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,” (Matthew 22:37-38).
•  “Jesus answered, “The foremost is, ‘hear, O Israel! The Lord our God is one Lord; 30 and you shall love the Lord your God with all your heart, and with all your soul, and with all your mind, and with all your strength,” (Mark 12:29-30).

Matthew uses three terms: heart, soul, and mind. Mark uses four: heart, soul, mind, strength.

“The difference may be accounted for on the assumption that Mark combined the readings of two manuscripts of the Septuagint, while Matthew had a preference for the three-membered form of the Hebrew text. In either case no distinction may be drawn between the meanings of the individual terms.¹Newman, B. M., & Stine, P. C. (1992). A handbook on the Gospel of Matthew. Originally published: A translator’s handbook on the Gospel of Matthew, c1988. UBS helps for translators; UBS handbook series (695). New York: United Bible Societies.

Jesus was quoting Deuteronomy 6:5 which says, “You shall love the LORD your God with all your heart and with all your soul and with all your might.”  But notice that he modified it a little bit and use the word “mind.” Consider the following quote about this.

“Jesus quoted the Septuagint²The Septuagint is the Greek translation of the Hebrew Old Testament done around 200-250 BC. almost verbatim, but he substituted mind (dianoia) for the similar-sounding “might” (dunameos). We are to take this list as an emphatic way of saying, “Love God with everything you are in every way possible.” But it was not without significance that our Lord deliberately substituted “mind” here rather than some other term. Christians need to take a lesson from this. We should learn to think critically and biblically.³Weber, Stuart K. Matthew. Vol. 1. Holman New Testament Commentary. Nashville, TN: Broadman & Holman Publishers, 2000.

I think it is worth addressing that Jesus utilizes the word mind. If Christians take this seriously, we are obligated to love God via our thinking and reasoning, not just our emotions.  We are to think of him and even use logic in discussing him and defending him with unbelievers. But Jesus’ quote encompasses the entirety of what we are as Christians. We are to love God with everything we are: heart, soul, and mind. Nothing is left out in these three (four if strength is included) encompassing terms.