"3Let not mercy and truth forsake thee: bind them about thy neck; write them upon the table of thine heart:" (Proverbs 3: 3)
When reading the Old Testament, it is crucial to bear in mind the Cross, which signals that God fulfilled the Old Covenant through the death and resurrection of His Son Jesus:
"For this is my blood of the new testament, which is shed for many for the remission of sins." (Matthew 26: 28)
"After the same manner also he took the cup, when he had supped, saying, This cup is the new testament in my blood: this do ye, as oft as ye drink it, in remembrance of me." (1 Corinthians 11:25)
"And to Jesus the mediator of the new covenant, and to the blood of sprinkling, that speaketh better things than that of Abel." (Hebrews 12: 24)
So, let us look for Jesus in the Old Testament, just as He revealed Himself to the disciples in Luke 24.
Whenever we read about "mercy and truth", we receive the revelation of Jesus:
"For the law was given by Moses, but grace and truth came by Jesus Christ." (John 1: 17)
Under the Old Covenant, man was required to come to God, to seek Him, and to establish his righteousness by what his actions.
Under grace, we do not seek God, but rather He seeks us:
"For when we were yet without strength, in due time Christ died for the ungodly." (Romans 5: 6)
"But God commendeth his love toward us, in that, while we were yet sinners, Christ died for us." (Romans 5: 8)
Once again, "The law was given", which imparts a sense of distance. "Grace and truth came" informs us that Jesus is up front and personal, moving into our lives, and giving us His life, Himself.
While King Solomon urged his readers to seek grace and truth, and bind it around their necks, under the New Covenant we can rest assured knowing that Jesus came and graced us with His truth, the Gospel of grace:
"And ye shall know the truth, and the truth shall make you free." (John 8: 32)
Paul connects the Gospel with grace and truth:
"Which is come unto you, as it is in all the world; and bringeth forth fruit, as it doth also in you, since the day ye heard of it, and knew the grace of God in truth:" (Colossians 1: 6)
We are full inheritors of this grace:
"3Blessed be the God and Father of our Lord Jesus Christ, who hath blessed us with all spiritual blessings in heavenly places in Christ: 4According as he hath chosen us in him before the foundation of the world, that we should be holy and without blame before him in love: 5Having predestinated us unto the adoption of children by Jesus Christ to himself, according to the good pleasure of his will, 6To the praise of the glory of his grace, wherein he hath made us accepted in the beloved." (Ephesians 1: 3-6)
Under law, we have to bind grace and truth to ourselves. Under grace, Jesus makes us bound for grace and truth through Himself.