I agree with you. Paths can be different but universal truth is only one. God is either one as Muslims claim or three persons in one being as Christians claim but both statements can't be true at the same time.
Sure they can. God's Holy Spirit permeates the creation, and Jesus came to embody Holy Spirit in the flesh, in order to give sinners a path back to God.
I understand what you are trying to say. Why you need holy spirit when God has already revealed the guidance?
There is nothing, not the Bible nor the Quran nor any other text, that provides a direct connection with God. Any text can be misinterpreted in an unholy way. All the scriptures and teachers/preachers/imams/prophets/etc. including Jesus, do is bring people to Holy Spirit so they can connect with God and serve Him directly.
If holy spirit guides each person differently then how you know the thing you are following is not from devil?
The devil is always trying to mislead/trick you. All you can do is earnestly seek Truth and be receptive to (true) Holy Spirit. When Holy Spirit reveals to you that you've been tricked/misled, you confess, repent, and learn from the mistake. No truth is revealed to your consciousness/conscience except through Holy Spirit.
Let's use a simple example to understand revelation through Holy Spirit vs. externally-validated truth: If a math teacher tells you that 2+2=4 or that Pi is an irrational number without repeating sequences of digits, you can accept the teacher's authority as an externally-validated source. In that case, you don't truly know whether the teacher is telling you the truth but you just accept it because you assume the teacher has better access to truth than you do.
If you are true to Holy Spirit, however, you will struggle to understand why/how 2+2=4 and/or how Pi can go on endlessly without repeating digits; and at some point you will experience revelation inside yourself that these claims are truly true. You no longer have to take the teacher or other authority-source's word for it that it's true because you have experienced knowing it for yourself.
This is not to say that we aren't sometimes deluded in believing something is true that later turns out to be false, but as long as we stay true to Holy Spirit, we are only interested in discovering further what is truly true or false, and not in defending knowledge that could turn out to be false. That doesn't mean we can't continue to honor the reasons we previously experienced false knowledge to be true, but only that we are always interested in becoming more aware of what is really true and how/why.
Why we needed prophets in the past if holy spirit is the way to guide you to the truth?
Prophets, angels, etc. are only true if their ultimate goal is to connect you with God directly. If their goal is to become an intermediary between you and God, that would be keeping you separated from God in their own selfish interest of controlling you. Think about it: when you teach your children something, is it because you want them to just take your word for it and never grasp it for themselves, or is it because you want them to see the truth for themselves and thus share in your direct experience of truth?
By the way, in Quran holy spirit refers to angel Gabriel who brings down Divine message from God to His prophets.
Yes, I remember you mentioning that. I don't know much about angels, except I just assume they are servants and messengers of God and that some are fallen and act as agents of evil/darkness instead. The story of Jesus' birth involves an angel:
18This is how the birth of Jesus Christ came about: His mother Mary was pledged in marriage to Joseph, but before they came together, she was found to be with Child through the Holy Spirit. 19Because Joseph her husband was a righteous man and was unwilling to disgrace her publicly, he resolved to divorce her quietly.
20But after he had pondered these things, an angel of the Lord appeared to him in a dream and said, “Joseph, son of David, do not be afraid to embrace Mary as your wife, for the One conceived in her is from the Holy Spirit. 21She will give birth to a Son, and you are to give Him the name Jesus, because He will save His people from their sins.”
I dont underestimate the value of Bible and Christian beliefs. I do agree that there is some truth in the Bible which can lead sincere people to ultimate truth.
Thank you for saying that. I see the human pursuit of truth somewhat like being in a stream with lots of rocks to step on so that you won't fall down and get swept backward by the current. Different rocks may come from different sources/religions/philosophies/etc. and some rocks may be more stable or slippery or bigger or smaller than others, but ultimately we all have to find our way upstream, sometimes getting swept backward, some giving up and just letting the current sweep them downstream until they finally realize they have to muster up strength again. I don't know if it is possible to ever give up completely and give up on the struggle, because I think there may always be a worse state farther downstream to motivate you to begin struggling toward the light again.