His life is eternal. As long as we CONTINUE to abide in Him we will continue to share His (eternal) life.If it can be lost it's not eternal.
When Jesus was praying to the Father in the garden this is what He said:
…2For You [Father] granted Him [the Son] authority over all people, so that He may give eternal life to all those You have given Him. 3Now this is eternal life, that they may know You, the only TRUE God, and Jesus Christ, whom You have sent.
(John 17:3)
Jesus did not define "eternal life" as merely "everlasting existence." Even those who are sent to Hell have that.
The word Jesus used for KNOWING the Father is ginṓskō (γινώσκω)which properly means, to know, especially through personal experience (first-hand acquaintance). It is this the kind of relational knowing Jesus was referring to
(https://biblehub.com/greek/1097.htm.)
There was another word for "know" and that was oida He could have used that but oida meant "to have seen or perceived, hence to know" That is not the way we come to know Christ (see: https://biblehub.com/greek/3609a.htm
You act as if one one step of faith guarantees that we will remain in Him forever. By contrast, the Bible says:
6Therefore, just as you have received Christ Jesus as Lord, continue to walk in Him, 7rooted and built up in Him, established in the faith as you were taught...(Colossians 2:6)
How did we first receive Him? First we saw our need for Him and we let go of our old life to walk in a dependent faith/love relationship with Him. That first step, Paul tells us must become a walk.