From All Eternity
When was the plan of redemption devised?
All our thoughts and imaginations will not alter in the least any part of the plan of redemption devised from all eternity. ST February 5, 1894, par. 6
If the Plan of Redemption was from all eternity - eternal, is it fair to conclude that there was never a time when it was not in existence? If there was never a time when it was not in existence, how come it be devised?
This is false human reasoning. The question is not a fair question, since it requires the one who answers it, to be able grasp the infinity.
Just because the Plan of Redemption is eternal, it doesn't mean it was never devised. To conclude that, it would be contrary to the testimony. Regardless that it seems contradicting, the Testimonies have authority. It says that there was a time before time commenced.
The plan of salvation, that expresses the love, the benevolence of Jesus Christ toward man was devised prior to the commencement of time. 7LtMs, Lt 27, 1892, par. 20
The plan of salvation, devised prior to the beginning of time, expresses the love of Christ to man, the devotion of the Son to the Father's glory. ST November 21, 1892, par. 8
We cannot understand the time before time, nor being eternal and having a beginning at the same time. How should we conduct ourselves in such regards towards the Testimonies?
The portions of Holy Writ presenting these great themes are not to be passed by as of no use to man. All that God has seen fit to make known we are to accept upon the authority of His word. Only a bare statement of facts may be given, with no explanation as to why or how; but though we cannot comprehend it we should rest content that it is true, because God has said it. All the difficulty lies in the weakness and narrowness of the human mind. 5T 699.4
Only by faith we can accept the idea that the Plan of Salvation is eternal, yet it is devised (having a conception, a beginning). We should not reason it out, rather accept it because it is sanctioned by the authority of the Word of God. The difficulty presented does not lies in the Testimony, rather in the weakness and narrowness of our human mind.
The same principle is applied to the existence of the Son of God. He is eternal, yet begotten in the express image of His Father's person.
Christ from all eternity
Christ is older than the Plan of Salvation:
The terms of this oneness between God and man in the great covenant of redemption were arranged with Christ from all eternity. ST August 24, 1891, par. 10
Christ is declared in the Scriptures to be the Son of God. From all eternity He has sustained this relation to Jehovah. Before the foundations of the world were laid, He, the only begotten Son of God, pledged Himself to become the Redeemer of the human race should men sin. 20LtMs, Ms 22, 1905, par. 4
Christ was one with the Father, from all eternity:
From all eternity Christ was united with the Father, and when He took upon Himself human nature, He was still one with God. ST August 2, 1905, par. 10
Christ was God essentially, and in the highest sense. He was with God from all eternity, God over all, blessed forevermore. RH April 5, 1906, par. 6
Christ had the glory from the Father, from all eternity:
As soon as this ceremony was completed, the Holy Spirit descended upon the disciples in rich currents, and Christ was indeed glorified, even with the glory which He had with the Father from all eternity. AA 38.3
"O Father, glorify thou me with thine own self with the glory which I had with thee before the world was" (John 17:5). This glory was given Him by the Father: "that they may behold my glory, which thou hast given me" (John 17:24).
See The Beginning Problem for more data that Christ is eternal yet has a beginning. TLDR: it is a mistery!
See The Order of Heaven - By the Bible study to examine the glory of Christ as the only begotten of the Father.