Question:In I Thessalonians 4:9, how were the Thessalonians "taught by God to love one another" so that Paul did not need to write to them? Are we taught in a similar way? Were other New Testament Christians taught in the same way? If so, why did apostles write to them telling them to love, I Peter 1:22, 3:8, etc?
"For what man knows the things of a man except the spirit of the man which is in him? Even so no one knows the things of God except the Spirit of God. Now we have received, not the spirit of the world, but the Spirit who is from God, that we might know the things that have been freely given to us by God. These things we also speak, not in words which man's wisdom teaches but which the Holy Spirit teaches, comparing spiritual things with spiritual" (I Corinthians 2:11-13).
All that Paul taught came from God. The very words that he used were the ones selected by the Holy Spirit to disclose the mind of God to mankind. That is why Paul stated, "If anyone thinks himself to be a prophet or spiritual, let him acknowledge that the things which I write to you are the commandments of the Lord" (I Corinthians 14:37). Paul confidently assured all that his teachings were not his or any other man's. "But I make known to you, brethren, that the gospel which was preached by me is not according to man. For I neither received it from man, nor was I taught it, but it came through the revelation of Jesus Christ" (Galatians 1:11-12).
Thus when Paul told the Thessalonians "But concerning brotherly love you have no need that I should write to you, for you yourselves are taught by God to love one another" (I Thessalonians 4:9), it is no different than when Jesus praised Peter for his observation. "Blessed are you, Simon Bar-Jonah, for flesh and blood has not revealed this to you, but My Father who is in heaven" (Matthew 16:17). Peter did not come to the conclusion that Jesus was the Son of God all on his own. Peter did not make it up. He saw the miracles and he heard Jesus' teachings, both of which Jesus said came from the Father, and simply acknowledged what he had seen and heard.
Paul is complimenting the Thessalonians for learning the lessons on brotherly love so well that he doesn't need to write to explain the concepts further to them. The Thessalonians had been taught both verbally and through letters. "Therefore, brethren, stand fast and hold the traditions which you were taught, whether by word or our epistle" (II Thessalonians 2:15). They also had Paul and his companions living among them and could draw upon their memories of their example. "For you yourselves know how you ought to follow us, for we were not disorderly among you; nor did we eat anyone's bread free of charge, but worked with labor and toil night and day, that we might not be a burden to any of you, not because we do not have authority, but to make ourselves an example of how you should follow us" (II Thessalonians 3:7-9).
Such should not be surprising. It was prophesied that Christians would come to God by learning. "But this is the covenant that I will make with the house of Israel after those days, says the LORD: I will put My law in their minds, and write it on their hearts; and I will be their God, and they shall be My people. No more shall every man teach his neighbor, and every man his brother, saying, 'Know the LORD,' for they all shall know Me, from the least of them to the greatest of them, says the LORD. For I will forgive their iniquity, and their sin I will remember no more" (Jeremiah 31:33-34). No one can become a Christian except by learning God's word. You can't be born a Christian. You cannot mysteriously become a child of God. You have to be taught. "How then shall they call on Him in whom they have not believed? And how shall they believe in Him of whom they have not heard? And how shall they hear without a preacher? ... So then faith comes by hearing, and hearing by the word of God" (Romans 10:14, 17). This is why you see people always being taught before becoming Christians. Even those for whom angels intervened still had someone come and teach them the truth. "No one can come to Me unless the Father who sent Me draws him; and I will raise him up at the last day. It is written in the prophets, 'And they shall all be taught by God.' Therefore everyone who has heard and learned from the Father comes to Me" (John 6:44-45).
By the way, love isn't the only topic that Paul said he did need to write further about. He also told them, "But concerning the times and the seasons, brethren, you have no need that I should write to you. For you yourselves know perfectly that the day of the Lord so comes as a thief in the night" (I Thessalonians 5:1-2). Again, he is complimenting them on learning this particular lesson well. He is not saying they learned about the second coming without ever being taught. He is only saying that they understand it so well there is nothing more he needs to write to them concerning it.
In regards to I Peter, you must recall that Peter was writing to a different group of Christians. While Paul said the Thessalonians had a good grasp on the subject of love, it doesn't follow that all Christians in all communities had an equally firm grip on the same topic.