I had heard eternity illustrated as a sheet of paper upon which the line of time is drawn, such that God looks on time all at once, beginning, middle and end, every moment of which is present to his gaze.
Here is another illustration of eternity which was new to me and also quite good, in my opinion:
There can be no adequate illustration of what transcends imagination, but the figure of the circle is helpful in comparing time and eternity. The circumference is subject to division and measure and yet endless to those who pursue its path. It is a fit symbol of time. The center, being a point, has no measure, and yet is abreast of every part of the circumference, however vast that may be. It symbolizes eternity.
The radii occupy various positions, and are drawn in different directions. But while these divergences produce different points of contact with the circumference, their points of contact at the center coincide under all circumstances. They symbolize the relations between the temporal and the eternal. Like the radii these relations are diverse and changeable, but the region of change lies in the temporal alone. When, therefore, we describe the relations of the eternal mind and will of God to the temporal effects of His operations in terms of temporal change and sequence, we are describing truly, if we remember that we are not describing the eternal center itself, which is immutable.
The Being and Attributes of God, Francis J. Hall, D.D., New York:Longmans, 1918, pp. 255-256.