Right. It's only eternal IF you abide IN CHRIST.If you stop believing and lose your salvation as a result, then the life you received wasn't eternal...it was only temporary.
John 15:1-6
Find out what an earnest is as in 2 Cor 1:22 and 2 Cor 5:5 which is YOUR verse...it means a pledge.....It has conditions.See Ephesians 1:13-14, 2 Corinthians 1:22, 2 Corinthians 5:5.
God gives the seal of the Holy Spirit to those who believe.
Therefore, because of His abiding influence, those who have this seal will not fail to keep His word.
Also Eph 1:13-14
It all speaks of a pledge, an earnest.
The promised item comes at the end.
Philippians 3:12-14
12Not that I have already obtained it or have already become perfect, but I press on so that I may lay hold of that for which also I was laid hold of by Christ Jesus.
13Brethren, I do not regard myself as having laid hold of it yet; but one thing I do: forgetting what lies behind and reaching forward to what lies ahead,
14I press on toward the goal for the prize of the upward call of God in Christ Jesus.