YES. Although I do not know how to approach the matter, by not remaining in the Catholic Church your son is cutting himself off from the grace of the sacraments, and without grace there is no salvation. Cite reference Catechism paragraph 846. Whatever social or other benefits his non-denominational church (ecclesial assembly) may offer, it has no Tabernacle and no Real Presence. Furthermore, many such churches can be subtly or openly hostile to bedrock tenets of Catholic faith and teaching. It depends on the individual congregation. More than the salvation of your son though this extends to and affects his wife and any children. Non-denominational churches often preach a symbolic communion, which implicitly denies the Incarnation. Your son cannot remain where he is without grave spiritual danger.
What you are saying is true in theory, but it is not true that this is the best thing to say to your young adult son or daughter who has already started moving away.
Find other true things to say that will be more persuasive to
them, but don’t say too much or you lose your credibility.
There may be wide variation in non denominational churches, so it depends a lot. Most of them have little doctrine, as opposed to evangelical or fundamentalist churches, which have very much doctrine.
Some Catholics join these churches because they feel the RCC is too legalistic. Quoting what you did confirms that prejudice. They say they need a “personal relationship with Jesus Christ” instead.
All you can do is gently, briefly, mention in passing as things come up in your life, how this Catholic belief or practice helped you in your own relationship with Christ, not telling them what they should do or believe.
If they ask you a question, give them a **brief **answer. If you give them long answers, especially telling them what **they **should do, there won’t be any future questions.
They have to know you love them now, and always, regardless of what they believe or practice. They will benefit from your religious experience at times, if they feel that love. But you won’t pick the times they will be open to your wisdom. You just have to love them all the time, be willing to listen all the time, and they will pick the times they will listen to you.
Don’t criticize their non denominational church. That hardens them in belonging to it. As times come up, you can **briefly **mention how and why you do something, but don’t elaborate unless they ask. And they likely will ask, eventually. If they feel comfortable with you, and know they are not opening the door to an hour lecture.
Pray.