The cross within a circle is an ancient symbol that you will find used by many ancient cultures and religions even to this very day.
You will find many adaptations of this cross such as the Celtic cross,
and The Cross of Jerusalem.
The most simple explanation would be that it is an ancient Gnostic symbol representing both the upper and lower halves of our world. The seen and unseen. The material and spiritual world.
The upper half, or as above would represent the invisible spiritual world which is derived both ancient and modern conceptions of heaven and the empyrean. It can also be construed as the soul and or lord.
The lower half, or so below represents the aspects of the visible world of creation such as the human who is composed of various chemical matters from the as above planetary spheres.
The point in the middle of the two halves united in the cross represents spirit and matter becoming one. The soul in union with a physical human thus creating a spiritual being.
This is also symbolized by the circle with the dot in the middle which is is sometimes called the circumpunct.
Also notice that both the lower and upper half are divided. The lower half divided, which signifies the division of Matter into both visible and invisible, animate and inanimate, organic and inorganic ; and the upper half divided, which represents Spirit divided into the (psychically) visible and invisible (higher) planes.
All these divisions united in one figure give us the Cross within the Circle or Sphere.
The goal for anyone who wishes to be initiated into the Gnostic secret mysteries is to unite the upper half being that of your soul and or lord representing the spiritual world with that of our physical body representing the material world.
Thus becoming a spiritual human who operates in the material world and is not ruled by matter and or beastly appetites.
The conqueror of that Old Dragon and Prince of the "world."
A divine human like Christ who is in control of his thoughts, actions and life.
Source: Rosicrucian Symbology