Theologians distinguish two kinds of eternity.
The eternity of God means existence with no beginning and no end. When the idea of eternity is applied to the angels and the saints in heaven, it is a limited concept in the sense that they all had a beginning but will have no end. They came into being by the almighty creative power of God, but their spirit has no intrinsic principle of corruption, as we find in material bodies, so they continue on in existence forever. Eternal life for a human being means that he will live forever, that his life will never end. This is something we all desire deep down in our hearts, whether we explicitly advert to it or not.