I started out planning to blog about something else (which I will), but I came across this historical information that will help you in your faith & reasoning development. In the past few postings I've mentioned how the Bible states that even foolish thoughts, and then of course obviously mean and unrighteous thoughts, are sins, even if no one takes action on them. I explained that and gave the scripture references in previous posts some time ago.
Well, I know that one of the things you need to think about is: how has accurate and complete understanding of the Bible diminished so much over the two thousand years of Christianity that my explaining the Bible has this information has become a news flash? Something that very, very, VERY few pastors and other moral leaders know about or if they do, even mention? Why does no one warn their flock that even foolish thoughts are considered by God to be sins?
Before you, or anyone, can answer that question, you need to "fact find." Young people (yes, hi there, I think of you fondly as always), the scientific method and the use of problem solving logic means that like a detective, you trace how far in history from the time of the Bible writings to the present that awareness of this particular admonishment in the Bible exists in both the religious and secular consciousness.
Thus I want to give you an example of how, while leafing through my prayerbook, I came across written evidence that just over three hundred years after Jesus Christ lived that people still knew full well and embraced the Bible teaching that foolish and sinful thoughts are actual sins. So while I'm not planning to spend time studying this for you, I thought, hey! What a perfect example to show you how to reconstruct how modern thought has gone so wrong. So the first step is to trace since the time of Jesus, using impeccable written factual sources (not imaginings of false prophets and 'psychics') what people actually thought and did regarding the topic that you are studying, in this case how the Bible states that foolish and sinful thoughts are sins, even if no actions follow.
Here is some background of the person I am going to quote. Ambrose was born around the year 320 AD (and thus was born nearly three hundred years after Jesus Christ was crucified and resurrected) into a upper class family in the Roman Empire. Ambrose's family had been Christian for several generations. Ambrose had in his family tree, in fact, a Christian martyr, St. Soteris. Ambrose and his family received classic legal education, so they were well educated and prepared for high civil office, so Ambrose became a lawyer and a governor. When a local bishop died, Ambrose was sent to help sort out arguments among the people about who should be appointed bishop in the place of the deceased. As he addressed the crowd on this subject a child in the crowd called out that Ambrose himself should become bishop! The crowd agreed and the two arguing sides with their respective candidates fell into agreement about Ambrose (who was shocked and did not want to be the bishop). He was well on his way up the lawyer and government "career ladder," and was still studying and deepening his own individual faith. But because the people wanted him so badly and basically drafted him, he was baptized, received the holy orders of priesthood, and was consecrated Bishop of Milan (Italy) all within a month of time! He was very obviously a pious and sanctifying man from the start (the people's instinct was correct!) He gave to the Church and the poor his considerable personal wealth, he sorted through the problem of the genuinely poor from those who were shams, he studied scripture and doctrine for twenty three years, was a prolific author and being a priest, as all bishops are, Ambrose celebrated Mass (the Holy Eucharist) every day. He even raised the three grandchildren of a friend.
With that as background, seeing this was a holy man who didn't even in fact seek out fame or a livelihood due to his sanctity, here is what he wrote, with his opinion of his own sinfulness! This is a long prayer he wrote that he recited before celebrating the Holy Eucharist (the "Lord's Supper") in daily Mass and I include here the opening of the prayer and the places where you see evidence of the realization of those times that all people are considered prone to sin, including of mere thoughts that are foolish or unworthy:
O Gracious Lord Jesus Christ, I, a sinner, presuming not on my own merits, but trusting to Thy mercy and goodness, fear and tremble in drawing near to the Table on which is spread Thy Banquet of all delights. For I have defiled both my heart and body with many sins, and have not kept a strict guard over my mind and my tongue....
...To Thee, O Lord, I show my wounds, to Thee I lay bare my shame. I know that my sins are many and great, on account of which I am filled with fear. But I trust in Thy mercy, for it is unbounded...
...Hearken unto me, for my hope is in Thee; have mercy on me, who am full of misery and sin, Thou who wilt never cease to let flow the fountain of mercy. Hail, Thou saving Victim, offered for me and for all mankind on the tree of the cross...
Remember, O Lord, Thy creature, whom Thou hast redeemed with Thy Blood. I am grieved because I have sinned, I desire to make amends for what I have done. Take away from me therefore, O most merciful Father, all my iniquities and offences, that, being purified both in soul and body, I may worthily partake in the Holy of Holies...
...I purpose to partake, may be to me the full remission of my sins, the perfect cleansing of my offences, the means of driving away all evil thoughts and of renewing all holy desires, and the advancement of works pleasing to Thee...
Do you see how often this obviously holy man (remember, the crowd proclaimed him when he had no clue of wanting to leave a high career for the sanctified priesthood) emphasizes and repeatedly confesses and asks for help for his sins of thoughts????
If you ever wonder what holy people confess to God, that is what it is. Truly holy and sanctified people continue to "renew all holy desires" by acknowledging that all humans have foolish, evil and sinful thoughts. This is the genuine humility before God that ALL believers should have, which is the acknowledgment that even the holiest of people have, due to being broken vessel human beings, regardless of their level of faith, have to mindfully struggle against having even silly and mean or vain thoughts, say nothing of how profound a sin that thinking, even idly and fleetingly, thoughts about sinful matters are.
So, young people, and others, this is how you can see that no, we don't "lack evidence" of "what the church was like" and "what people believed" "back then" in "Biblical times." This lawyer/governor who became priest/bishop only a few hundred years after Christ left plenty of written evidence of his thoughts and what the people believed, and what they retained of their understanding of the Bible, both Old and New Testament. The notion that even idle, foolish and sinful thoughts are actual sins (as the Bible states) was a hot forefront belief several hundred years after Christ.
Thus, young people, and others, you now have a piece of your investigation, if you were doing so, that for certain in the four century that the knowledge that bad and foolish thoughts are sin was well known and frequently meditated about and prayed regarding, including DAILY by this Bishop of Milan, Ambrose.
Those of you new to studying the saints, let me explain that they were not tucked away in a corner. Ambrose, while he was living, was studied by many who would become great saints themselves. Further, while there was no email, ha, or post office, there was indeed snail mail and these people were all in correspondence. So if you continue your investigations you will see that a writing by a priest or bishop was never a "personal" so called "interpretation of scripture." This was the prevailing belief, kept intact from Old Testament times and the times of Jesus, that foolish and bad thoughts are indeed sins.
I hope that you have found this helpful both in further understanding what I have been reminding people about what the scriptures actually say, but also as you can see that you don't have to imagine and make stuff up, but you can look at written agenda-free evidence to deduce what people knew and believed, and how the word of God is preserved, and when parts of it started to fall out of public consciousness. Here we have filled in the blanks of several hundred years, knowing that the early Christians were keenly aware of what the Israelites knew and what those who followed Jesus knew, which is that God states that foolish, bad, mean and/or sinful thoughts are indeed actual sins, whether actual action follows or not.