The word eternity has several meanings. For example, when we speak of someone dying, we sometimes say that they have “passed into the presence of the Lord”. Most often, we hear people say that the person has “passed into eternity”, and so it has come to mean the future world rather than this present one. From a theological standpoint, eternity is an aspect of His infinity and it is related to our concept of time. When we say that a person is eternal, we mean that they do not have a beginning or an ending. This is not endlessness, however, because every person and creature has a definite point at which their life on earth will cease and their never-ending life in either Heaven or Hell will start. In this sense, we all possess eternal life, but not all in the knowledge and life of God, for all of us shall live forever.
Our state of existence will be determined by whether God’s life is inside or not. The location of someone’s eternal existence is what separates the saints from the unbelievers. Nevertheless, both shall live forever because all will have an endless existence. However, this is not eternity, because when we say that God has eternity, it means He has no beginning or ending as humans possess. So, eternity is a reference to the fact that God is an infinite being with respect to time. Psalm 102 gives us a beautiful description of the eternity of God.
Of old hast thou laid the foundation of the earth: and the heavens are the work of thy hands. They shall perish, but thou shalt endure: yea, all of them shall wax old like a garment; as a vesture shalt thou change them, and they shall be changed: But thou art the same, and thy years shall have no end. (Psalms 102:25-27)
The Psalmist gave us a picture of the whole creation of God. He thought of the heavens and the earth, and as he looked, he noticed that they began to get old and were finally folded up and dispensed. This creation, which has had its beginning and its presence, has fully aged. It is finally done away with. Yet, there stands the eternal One, who is from the beginning, always is, and has an endless future. He is an eternal being and this is because of the very fact that He exists of Himself and is not dependent upon anyone else.
If there had been a time when God had a beginning, then there must have been a time when He was not self-existent. Since He is self-existent, He must be an eternal being. His eternity is determined by self-existence, which is His immutability. The fact that God is a changeless being means that He could not have had a beginning and He must not have an ending.