A baby can't do anything for anybody even himself, can't even feed himself without Mama sticking it right there into his mouth.
Babes of the spirit --who are SAVED but usually, or at least often, not yet capable of bearing much discernible fruit. Sometimes you can't tell them very much apart from the unsaved.
Not for nothing Jesus used child-likeness as a pre-requisite for the kingdom of Heaven. They need the "sincere milk" of the word to grow. As they learn and practice flailing and keep getting the word they'll grow and later on they can feed on the meat of the word.
False doctrines are fruits of feeding meat to babes before they're ready, but mostly because of sin of course. Some babes choke on the meat.
If John 3:16 is a promise from God of eternal life upon believing on Jesus Christ. If you "lose it" upon your next sin, then what kind of "eternal" is that??!!!
Obvious answer: It isn't!! Duh. For ALL have sinned and come short, and if we say we have not sin we are liars and make God a liar.
And we know God is not a liar, therefore by the laws of logic and the axiom of the word of God we know that once one believes in Jesus Christ, he has eternal life, he is a new creature, [Jesus] must increase and I must decrease.
2 Timothy 3:15 KJV