God's love is actually expressed in law. The great principle of law is love. Therefore the principle of the gospel itself is expressed in Deuteronomy.
...God so loved the world, that he gave his only begotten Son... (John 3:16)
You and I express our love for God in our obedience. The Lord Jesus put it like this:
If ye love me, keep my commandments. (John 14:15)
This is still the acid test today. If we love Him, we will keep His commandments. Salvation is a love affair.
We love him, because he first loved us. (1 John 4:19)
The Lord Jesus cited this as the greatest commandment of all:
And thou shalt love the LORD thy God with all thine heart, and with all thy soul, and with all thy might. (Deuteronomy 6:5)
Our obedience is the manifestation of our love. Obedience is the important thing all the way through — it is "if they keep these commandments." Now you may wonder what is new about love in the New Testament if love is in the Old Testament. The difference is that in the New Testament the love of God has been translated into history by the incarnation and death of Christ.
But God commendeth his love toward us, in that, while we were yet sinners, Christ died for us. (Romans 5:8)
He died for us! You see, it is one thing to express love by bringing Israel out of Egypt; it is another thing to die for them. It is one thing to say something from the top of Mount Sinai; it is another thing to come down and take our frail humanity upon Himself, to be made in the likeness of Man, and to die on a cross for our sins. I repeat, salvation is a love affair.
Herein is love, not that we loved God, but that he loved us, and sent his Son to be the propitiation for our sins. (1 John 4:10)
—From Edited Messages on Deuteronomy by J. Vernon McGee