God’s deliverance of the Israelites with “a strong hand and an outstretched arm” is to be recalled by means of “a sign on your hand and a reminder between your eyes” (Ex. 13:9).
The tefillin are attached to the person of a male Jew to indicate that he is bound to God, to the Divine commandments and to Jewish history.
The tefillin are obligatory upon every Jewish male, hopefully for all three reasons. But there are different categories of Jews.
Some live a Jewish life because of belief; for them the tefillin are a symbol of love for the Almighty.
Some whose faith is not so strong live a Jewish life because Jewish observance is a mark of Jewish identity; for them the tefillin are a symbol of the pattern of commandments and practices that make Jews distinctive.
Some are strong neither in faith nor in practice but they still have a bond with Jewish peoplehood; for them the tefillin are a connecting link with national history.
Every Jew is precious, every Jew is crucial, every Jew is an asset. Even if some are not (yet) ready for full spiritual commitment, whatever they do as Jews and for Judaism is valuable to them and valuable to the Jewish heritage.