A Church needs both simple gospel basics and deeper truth -- and I think that both young Christians and mature Christians need both. Here's an example of a simple message I delivered one Christmas, to mature Christians:
• Christ was God's revelation, andI quoted appropriate Advent verses in support, and applied it by saying that we the Church therefore need to be centred in God's revelation, and hold out God's salvation. OBSERVATION: I am thankful for my own salvation to a professor, Herbert Jantzen (see the German Wikipedia -- he is now 95 years of age). He taught us complex theology -- yet regularly changed gear, and spoke to us as if we knew nothing.
• Christ was God's salvation.