The lion and the lamb are both used symbolically in the Bible to illustrate the opposite qualities that make up both God and human beings. These contrasting qualities of strength and ferocity (lion) and gentleness and docility (lamb) also also to relationships between human beings and God.
Both animals appear many times in the Bible. The oft-quoted phrase "the lion shall lay down with the lamb" is actually an amalgamation of two passages in the Bible from the Book of Isaiah, verses 11:6 and 65:25. Both of these passages describe what the world will be like after the return of Jesus to the earth as predicted in the Bible.