I was taught that it is correct to use the holy water font to make the sign of the cross only as we enter the church, not as we exit. Is this true?
The use of holy water fonts derives from the Jewish practice of purifying oneself before entering the temple (Lev. 12-15). Holy water fonts located at the entrance of churches are meant to remind us that we were purified in baptism and to seek forgiveness (“re-purification” so to speak) of our post-baptismal sins.
It is certainly most appropriate, and traditional, to bless ourselves with holy water upon entering a church, but there is no absolute requirement that we do so. There also is nothing that states that we cannot or should not use the holy water on the way out of the church. While it would not be the same symbol of “entering the temple,” it could be used to remind us of our baptismal calling to live the gospel in the world.