The shape of the sign is a reminder of the cross of Christ. Historically, the sign has also been viewed as representing the trinity: Father, Son, and Holy Spirit. Through faith in the Lord Jesus Christ and His substitutionary death on the cross, salvation is extended as a free gift to all of humanity. read more
And God said, This is the token of the covenant which I make between me and you and every living creature that is with you, for perpetual generations:
And it shall be for a token upon thine hand, and for frontlets between thine eyes: for by strength of hand the LORD brought us forth out of Egypt.