We can see that these "some" among them had no concept of resurrection in the question;
1 Corinthians 15:35 (LEB) But someone will say, “How are the dead raised? And with what sort of body do they come?”
Exactly. Paul uses the fulcrum point of "true" Christianity, that being "the resurrection" of Jesus AND the promise of resurrection for believers as a basis of exposition of numerous matters of interest in 1 Cor. 15. It was not a fulcrum of teaching non-OSAS, but an exposition of Spiritual understanding.
Holding fast to the Word or to the Gospel can mean and encompass a lot of things, meanings, Words. Technically, when we "hold fast" to the Gospel, we "hold fast" to every Word of God. We understand for example, from Heb. 12:4, that "The Word" is both Alive and Active. That Word is Jesus, the Living Word.
The Word tells us everything we need to know. And The Word reveals many things to us, about ourselves. Some things, unpleasant. Do we "hold fast" to them?
Maybe we just hold fast to the Words we are inclined to hear? And discard the Words we don't like or care to hear.
Taking on the whole counsel of God in Christ, in His Words, is a life long venture, for "some." But we should understand that there are different levels of pursuit or following the Word. Some (well all believers technically) for example are "congregation." Some, continuing on are disciples. Some, continuing on are teachers. Some, continuing on are prophets or Apostles. (I don't know if I've really met any, not that I'm aware of anyway.)
But there are different stages of "growth." As God Sees Fit and Directs. None of us should be led to cast down the faith, however small, of anyone. Or to cast stumbling blocks in their ways. It might seem to me that not encouraging people in their faith, and rather, telling them that by their own power, they MUST pursue or else, is a stumbling block that does not TRUST God in Christ to lead them all.
I trust God in Christ leads all believers, however faint, however strong. To attempt to cast them down is simply not an option. We all should have more faith in Christ to lead us all, however small of steps we take. These steps are not directed by "the believer" but our Maker.
In Christ we are led to make FOLLOWERS, disciples, of Christ. Why would we take any such and cast them aside, or threaten them? The intent is unto eternal life. A non-OSAS view heads in the opposite direction. We don't look back.
Let's look at what Jesus Does:
Isaiah 42:16
And
I will bring the blind by a way that they knew not;
I will lead them in paths that they have not known:
I will make darkness light before them,
and crooked things straight.
These things will I do unto them, and not forsake them.