A cross anchor tattoo refers to the incorporation of both the symbol of a cross and that of an anchor into a complete design. The significance of this design hearkens back to those who used the image of an anchor to camouflage and hide the image of the cross. Some believe that this was done in order to avoid religious persecution while still being able to express one's faith.
The symbol of the anchor has been affiliated with Christianity since the time of Jesus Christ. During this time, it was common for Christians to be persecuted for their religious beliefs. The cross disguised as an anchor therefore became a symbolic image of secret religious expression. In addition to the cross itself, the anchor also has religious connotations, as it normally is associated with the ocean. Many of the people mentioned in the Bible are connected to the sea, and in turn the anchor.
In addition to ties with the Bible, Jesus and his disciples, the anchor is also commonly associated with Saint Clement of Rome, who is known to have been the Bishop of Rome during the late second century. Clement was executed by the Roman Emperor Trajan by being tied to an anchor and cast into the sea.