"Thou shalt love the LORD thy God with all thine heart, and with all thy soul, and with all thy might" (Deuteronomy 6:5).
"Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind, and with all thy strength: this is the first commandment" (Mark 12:30).
The "first commandment" to love the Lord our God with all our heart, soul, mind, and strength is not repeated in the New Testament epistles, and after the cross, the resurrection, and the giving of the indwelling Holy Spirit on the day of Pentecost.
The reason for this is that after this saving grace of the Lord Jesus became available to human hearts, we are now to love the Lord our God with all His heart, soul, mind, and strength.
"The love of God is shed abroad in our hearts by the Holy Spirit, which is given unto us" (Romans 5:5).
"I live, yet not I, but Christ. And the life I now live, I live by the faith of the Son of God, who loved me and gave Himself for me" (Galatians 2:20).
"By the grace of God I am what I am: and His grace which was bestowed upon me was not in vain; but I labored more abundantly than they all: yet not I, but the grace of God which was with me" (I Corinthians 15:10).
The human faculties of our heart, soul, mind and strength are still engaged, of course, and even more so because they are energized by the "exceeding, abundantly above" power of the risen Lord Jesus (Ephesians 3:20). Thereby we love God in a manner not possible before the gift of the permanently indwelling Holy Spirit was given to all who believe in the Savior. We love God as enabled by God, but nevertheless in a manner that returns to Him hued and fragranced by our individual person and personalities, as uniquely designed by Him.
There is no greater joy than to love our Heavenly Father through the dynamic presence of the Spirit of His Son, "sent forth... into your hearts" (Galatians 4:6). We were made and redeemed for this, and eternity will involve the continual knowing and receiving of the love of God, and the continual returning of it to Him by the Holy Spirit's joyfully leading and enabling. Wonderfully, it is the answer to a prayer, the sublime request of the Lord Jesus in His high priestly prayer of John 17, uttered just before the cross that made the answer possible...
"Father, I will that they also, whom Thou hast given Me, be with Me where I am; that they may behold My glory, which Thou hast given Me: for Thou lovedst Me before the foundation of the world. O righteous Father, the world hath not known Thee: but I have known Thee, and these have known that Thou hast sent Me. And I have declared unto them Thy name, and will declare it: that the love wherewith Thou hast loved Me may be in them, and I in them" (John 17:24-26)