Ultimately, of course, but we're talking about the life of the saved person:
"The only thing that counts is faith
expressing itself through love." - Galatians 5:6
"
17For the kingdom of God is not a matter of eating and drinking, but of
righteousness, peace and joy in the Holy Spirit,
18because anyone who serves Christ in this way is pleasing to God" - Romans 14:17-18
But osas would have you believe that what matters is that you know and accept the doctrine of osas. There's nothing evenly remotely true about that. You can take comfort all day long in what you know but if you don't have........well, we'll let Paul say it:
"2If I...can fathom all mysteries and all knowledge, and if I have a faith that can move mountains, but do not have love, I am nothing." - 1 Corinthians 13:2
But osas makes it's followers think that believing in the doctrine about osas is what it means to really know God and please Him. Not knowing that thinking actually takes away from what really matters in the life of the Christian. It provides a false sense of comfort and assurance by directing attention away from that which actually gives comfort and assurance--how you act--and to that which does not.
That's why we should continue to believe!
We know from the whole counsel of scripture that as long as the deceived believer responds to God's correction they will continue in the surety of Christ. Much depends on how deeply rooted the Word of God is in a believer.
The good and noble soil that allows the Word to be firmly and deeply rooted in it is the soil that perseveres in fruitfulness. While the believers with the shallow, rocky soil in which the Word can not be firmly and deeply rooted "believe for a while, but in the time of testing they fall away." (Luke 8:13).
That doesn't mean, categorically, that all 2nd type of soil believers fall away, or fall away and don't come back. It means that is the kind of soil for which that is likely to happen, i.e. the Galatians. No immature, 2nd type of soil believer has any claim to some kind of osas status. If anybody does it is the mature 4th kind of soil believer.