A lot of this hinges around the doctrine of "once saved, always saved" (Calvinism TULIP), aka "eternal security". People fail to see the harmony between 1 John 3:8-9 and 1 John 1:8-10 and 1 John 2:1 because of the preconceived notion that "once saved, always saved" is true. Thus, these verses appear to contradict.
The same scenario is affected in Acts 8 with the Samaritans and Simon. In Acts 8:5-24, we read of Philip (one of the seven, Acts 6:5) going to Samaria. In vs. 12, we read the Samaritans believed and were baptized. Likewise, Simon did the same in vs. 13. If the Samaritans were not saved, neither was Simon. If the Samaritans were saved, so was Simon.
But, Simon fell into sin (vs. 18-21). A Christian who sinned. And the apostle Peter told him to repent (vs. 22-24). Yes, Simon obeyed the gospel. He was in a saved condition...but after sinning, Peter told him to repent and pray to God that he might be forgiven.
Calvinism is false doctrine. Those who teach/believe it will say of Simon, "he was never saved in the first place". Why do they say this? Because it contradicts their false doctrine. Read for yourself -- if the Samaritans were saved in vs. 12, then so was Simon in vs. 13. If Simon was not saved in vs. 13, neither were the Samaritans in vs. 12.
God is speaking to us through His divine word showing Christians that it is possible to fall from grace. In fact, Galatians 5:4 affirms this possibility (the context there speaking of Jews clinging to the Mosaic Law). One cannot "fall from" grace unless once in grace. Speaking of Christians (brethren), James shows that one can depart from the truth in James 5:19-20 (notice there, the one who departs is called a "sinner"). How can one depart something they were not once in? When we abide in Christ, walk in the light, and are obedient to God, nobody can snatch one from God's hand (John 10:28) ...but one certainly can depart on his/her own. We have free will. Even Paul said this of himself (1 Cor. 9:27), knowing the possibility of apostasy.
Further, Peter uses the illustration in 2 Peter 2:20-22 showing one can first escape worldliness, then obey the gospel to become a Christian, and fall back into the sin of the world. Vs. 21 says, "For it would have been better for them not to have known the way of righteousness, than having known it, to turn from the holy commandment delivered to them".
The advocates of Calvinistic doctrine "once saved, always saved" (and also "total hereditary depravity", aka "inherited sin") cannot harmonize these scriptures in 1 John because they believe Calvinism to be true. It is complete error. It never fails, that someone who believes Calvinism may point to an indvidual and say, "He is a saved man". But, if that man falls into a publicly known sin (i.e., adultery), then they will say of the same man, "That man was never saved in the first place!"
False teaching always contradicts. Study these examples in your own Bible -- about Simon in Acts 8 and what the apostles' said on the matter -- Paul and Peter, as shown above.
To obey the Lord, one must hear the gospel (Rom. 10:17), have faith (Heb. 11:6), repent of one's sins (Luke 13:3), confess faith in Christ (Rom. 10:9-10) and be immersed into Christ (Mark 16:16; Rom. 6:3-6) = then, and only then, does the Lord add the saved to His church (Acts 2:47) of which He is Savior (Eph. 5:23).
If we sin, we have an advocate with the Father, Jesus Christ (1 John 2:1). We must confess this sin to God (1 John 1:9) and repent of the sin (just as Peter told Simon, Acts 8:22). Remember, the apostles' were guided into all truth by the Holy Spirit (John 14:26, 16:13). These are the inspired words of God.
There is no contradiction in 1 John. The Bible harmonizes. God be with you all!