[annalex]

If the image of the sheep were meant to convey the eternal security of the sheep even form itself, then how come Christ speaks of lost sheep?

"the Son of man is come to save that which was lost."

[blue-duncan]

"then how come Christ speaks of lost sheep?"

His sheep were lost until He came. That's the point of His coming. When He gathers the last that are His, He will come again as King and Judge, but in the meantime, all that He has gathered will not be lost again or taken from Him.