The Supreme Lord is known as Shaasvatah, meaning He who is Everlasting.
The Lord is known to have the Sat-cid-ananda Rupam where Sat- means eternal. So, His form is always present and is everlasting. The Lord appears in several avataras or forms in this world to protect His devotees and to Kill/vanquish the demons/asuras/anti-social elements. He appears in one avatar after another and shows He is everlasting or eternal.
paritranaya sadhunam vinasaya ca duskrtam
dharma-samsthapanarthaya sambhavami yuge yuge (BG 4.8)
In order to deliver the pious and to annihilate the miscreants, as well as to reestablish the principles of religion, I advent Myself millennium after millennium.
The Lord as the controller of Time, who is Time Himself never changes and so He is eternal. Unlike us the material beings who are bound by Time
Lets chant this Holy Name and seek to be situated in His devotional service eternally. For we are nothing but the servant of the servant of the servant.
Nama 57 – VishnuSahasranama