Deuteronomy 6:8-9 And thou shalt bind them for a sign upon thine hand, and they shall be as frontlets between thine eyes. And thou shalt write them upon the posts of the house, and on thy gates.
Sinners need reminders of the Gospel, for we are liable to forget the grace and the mercies of God. It is all too easy to become distracted by the world and life’s daily worries that we forget God in all of it. Jesus said to Martha, “you are anxious and troubled about many things, but one thing is necessary (listening to Jesus).” Jesus also say in the Parable: “the cares of the world and the deceitfulness of riches choke the word.” It is surprisingly easy to forget and drift.
Therefore, we respond to God’s goodness by setting up various kinds of reminders in our lives “lest ye forget.” The Jews responded literally by making phylacteries, which were little boxes with the words of Deuteronomy 6:4-5 written inside. These were attached to the forehead and were “frontlets between the eyes.” Jesus scolded the Pharisees for their pride and presumption in making them “long” and ostentious so people would notice their piety.
Christians today wear crosses as jewelry and hang up religious plaques, which is a good thing to do, but it still misses the point. The point is to be reminded by hearing the gospel as often as possible, at least weekly and daily, and then praying without ceasing during all opportunities of the day. We don’t do this to show off our goodness but the opposite: we do so precisely because we are sinners who constantly need grace. We find ways to remind ourselves of Sin and Grace.