The answer to the first question is somewhat complicated, but the second can be answered easily and correctly with an emphatic No. Intellectual powers do not wane from one generation to another. We are as smart as our forefathers. Any thought they could entertain we can entertain if we are sufficiently interested to make the effort.
The major cause of the decline in the quality of current Christian literature is not intellectual; it is spiritual. To enjoy a great religious work requires a degree of consecration to God and detachment from the world that few modern Christians have experienced. The early Christian Fathers, the mystics, the Puritans, are not hard to understand, but they inhabit the highlands where the air is crisp and rarefied and none but the God-enamored can come.
Rather than climb the mountain we choose to dig our shallow caves a few feet above the floor of the valley. Our spiritual moods and emotions are degraded. We eat and drink and rise up to play. We take our religious instruction in the form of stories, and anything that requires meditation bores us. And writers and publishers contribute to our delinquency by providing us with plenty of religious nothing to satisfy our carnal appetite.
O ye Americans, our mouth is open unto you, our heart is enlarged!
"Give attendance to reading" (1 Timothy 4:13, KJV).
Verse The mind of the sinful man is death, but the mind controlled by the Spirit is life and peace; Romans 8:6
Thought Control of our minds by the Spirit is encouraged as by reading we expose ourselves to the lives and thoughts of those who have walked with God. They have experienced His presence both in the mountains and valleys of life and there are lessons for us in their footprints.