Salvation is of the Lord, not of man's will or works; so, of course it is eternal. We are given eternal life, not eternal probation!
John 10:25-30 (W.E.B.)
25 Jesus answered them, “I told you, and you don’t believe. The works that I do in my Father’s name, these testify about me.
26 But you don’t believe, because you are not of my sheep, as I told you.
27 My sheep hear my voice, and I know them, and they follow me.
28 I give eternal life to them. They will never perish, and no one will snatch them out of my hand.
29 My Father, who has given them to me, is greater than all. No one is able to snatch them out of my Father’s hand.
30 I and the Father are one.”
1 Peter 1:3-5 (A.V.)
3 Blessed be the God and Father of our Lord Jesus Christ, which according to his abundant mercy hath begotten us again unto a lively hope by the resurrection of Jesus Christ from the dead,
4 To an inheritance incorruptible, and undefiled, and that fadeth not away, reserved in heaven for you,
5 Who are kept by the power of God through faith unto salvation ready to be revealed in the last time.
1 John 3:9 No-one who has been born of God practises sin, because His seed abides in him; and he cannot keep on sinning, because he has been born of God.