NOTE. Parts of my answer is in the Shia perspective, other Muslims may not agree upon them.
- First. Is Islam the religion that I want to switch to?
Perhaps a main distinction of Islam compared with its previous versions (mainly Judaism and Christianity) is that its prophet's miracle (The Quran) is still with us. Anyone can read it, think about it, ask about its ambiguous verses and eventually decide by himself whether to accept this claimed miracle as a real miracle and believe in Islam or not. Be warned, there are some ambiguous verses intentionally placed inside the book; see [3:7] for announcing this news.
This simplifies accepting Islam in comparison to accepting the other heavenly religions, since no Christian can ever claim that he can show you the miracles of Jesus --peace be upon him-- that prove he was a prophet (being a God is superior to being a prophet of course!). Also no Jew can claim a similar thing with respect to Moses --peace be upon him--, but all Muslims can make such a claim. It is somewhat like living in the time of the holy prophet, peace be upon him. There are differences as well, since no one of us know The Quran as well as it was understood by the messenger himself. The prophet's successors, The Fourteen Infallibles according to Shia Muslims, are considered as exceptions as they were taught be the holy prophet himself whatever was required for guiding the humanity. However, the fourteenth of them Imam Mahdi peace be upon him is hidden in this era, not accessible by all. Thus your questions may either be answered or left unanswered when are asked from us, the Muslims.
The Quran, like almost all other beings created by Allah, is layered in the sense of truth. It has one face and many layers of truth hidden inside. Behind the face, each layer of truth is covered by an upper layer of truth. The gradual simplification of The Quran from its utmost real being to its face available and understandable to many believers and non-believers is called the revelation of The Quran (تنزیل القرآن). The truth itself is in no language; it is basically a light of knowledge, but its revelation is in Arabic, as it is the most precise and natural language, and therefore the best choice to clarify, albeit in an encrypted manner, the truth in alphabets and within a rather small number of pages.
Each verse in The Quran is called a sign (Ayah: آیة) and acts as a door to the deeper layers of truth that Allah has meant to show us. The way to open this door is through the heart and by being clean and purified. The cleaner and more purified one is, the more deeply one can travel inside the verse and the more one can touch its truth meant by Allah:
إِنَّهُ لَقُرْآنٌ كَرِيمٌ \ فِي كِتَابٍ مَّكْنُونٍ \ لَّا يَمَسُّهُ
إِلَّا الْمُطَهَّرُونَ \ تَنزِيلٌ مِّن رَّبِّ الْعَالَمِينَ \
أَفَبِهَـٰذَا الْحَدِيثِ أَنتُم مُّدْهِنُونَ \ وَتَجْعَلُونَ
رِزْقَكُمْ أَنَّكُمْ تُكَذِّبُونَ
That this is indeed a qur'an Most Generous [that is, open handed], /
In Book well-guarded [that is, highly encrypted], / Which none shall
touch but those who are clean [touching its face allowed only by
having Vozu', and touching its deeper truth only by having clener
hearths] / A Revelation from the Lord of the Worlds. / Is it such a
Message that ye would hold in light esteem? / And have ye made only
this your livelihood that ye should declare it false? [while you could
exploit its messages many times more by accepting them!] [56:77-82]
See here for a probable explanation of the layered structure of the universe Allah has created.
As already explained, the book, although being much simplified, is still mysterious and ambiguous in parts. Indeed, this is the very intention of Allah. We should go to His men, those firmly grounded in knowledge (which means they should be purified more than everyone else), for them to better clarify the situation for us, [3:7]:
هُوَ الَّذِي أَنزَلَ عَلَيْكَ الْكِتَابَ مِنْهُ آيَاتٌ مُّحْكَمَاتٌ
هُنَّ أُمُّ الْكِتَابِ وَأُخَرُ مُتَشَابِهَاتٌ ۖ فَأَمَّا الَّذِينَ
فِي قُلُوبِهِمْ زَيْغٌ فَيَتَّبِعُونَ مَا تَشَابَهَ مِنْهُ ابْتِغَاءَ
الْفِتْنَةِ وَابْتِغَاءَ تَأْوِيلِهِ ۗ وَمَا يَعْلَمُ تَأْوِيلَهُ
إِلَّا اللَّـهُ ۗ وَالرَّاسِخُونَ فِي الْعِلْمِ يَقُولُونَ آمَنَّا
بِهِ كُلٌّ مِّنْ عِندِ رَبِّنَا ۗ وَمَا يَذَّكَّرُ إِلَّا أُولُو
He it is Who has sent down to thee the Book: In it are verses basic or
fundamental (of established meaning); they are the foundation of the
Book: others are allegorical. But those in whose hearts is perversity
follow the part thereof that is allegorical, seeking discord, and
searching for its hidden meanings, but no one knows its hidden
meanings except Allah and those who are firmly grounded in knowledge
say: "We believe in the Book; the whole of it is from our Lord:" and
none will grasp the Message except men of understanding.
Since Allah wants us to stay on their door, they are the most purified people as no one can access the concepts hidden in Quran but those who are clean. The cleaner and more purified one is, the more one can understand of this encrypted book. So people who reject going to the door of those firmly-grounded-in-knowledge, to ask them, learn from them, accept from them and finally obey them, are rejecting staying on the doors of those who are the most purified humans ever. This is due either to their lack of knowledge, or to arrogance and/or jealousy. In the latter case, they will arise in discord:
اللَّـهُ وَلِيُّ الَّذِينَ آمَنُوا يُخْرِجُهُم مِّنَ الظُّلُمَاتِ
إِلَى النُّورِ ۖ وَالَّذِينَ كَفَرُوا أَوْلِيَاؤُهُمُ الطَّاغُوتُ
يُخْرِجُونَهُم مِّنَ النُّورِ إِلَى الظُّلُمَاتِ ۗ أُولَـٰئِكَ
أَصْحَابُ النَّارِ ۖ هُمْ فِيهَا خَالِدُونَ
Allah is the Protector of those who have faith: from the depths of
darkness He will lead them forth into light. Of those who reject faith
the patrons are the evil ones: from light they will lead them forth
into the depths of darkness. They will be companions of the fire, to
dwell therein (For ever). [2:257]
With this short guide about The Quran, you may start a survey about this claimed miracle, then if you decide to convert, you should follow the next step.
- Second. What should I do to become a Muslim?
When a non-Muslim decides to convert and become a Muslim, then they must confess to two things: that Allah is God and the only god, and that Muhammad is his messenger (اشهد ان لا اله الا الله و اشهد ان محمد رسول الله). This is the beginning of becoming a Muslim and the final stage of being a Muslim as well, depending on how much the concepts are understood and believed by the person and applied to his everyday life. This has been already addressed elsewhere.
- Third. How do I stay a Muslim?
The first thing to do (which may take days, months, or even years, but anyway it is a must-do for staying a Muslim) is to understand and completely admit the pillars of Islam, the most fundamentals belief in Islam. This is completely on your own shoulder; you cannot accept any of them unless you understand and admit them by yourself. Following the scholars is not permitted in this stage. Then (second in rank but not in timely order) you should do the duties commanded by Allah, as you have already chosen to submit to Him, if and when you have passed the two steps mentioned above this paragraph. These duties, their details and sometimes even a whole duty, may vary from sect to sect, so you should choose one and follow its rule. After you have chosen your sect, you will no longer be expected to learn why you should do what, although it is always good that people know the wisdoms behind what they are expected to do, so as to help them do what they do better.
Sectarianism is not a real part of the religion but like the pillars of Islam you should have a reason for yourself why to choose which. Otherwise you would always feel unsafe and uncomfortable, wondering if the one Sect whose rules you are following is not the true Sect and whether how you pray to God is not the way that is really expected from you. This is the reason why I asked this question and hope that a Sunni brother will answer it; also I pray that the Sunni brothers would not consider it an attempt to increase the sectarianism, as I think that at Judgement Day, all of us should be able to rigorously explain the reasons behind our deeds. If we are expected to do whatever Allah has commanded us, we should first make sure what we are told is really what Allah has commanded, so that we always have reason for ourselves why we are doing such. [maybe not bad at this stage to take a look here and here]
This is the way to stay in Islam. Rebelling against a rule of Islam, after we have understood it being a command of Allah, is to explicitly disobey Allah, which is equivalent to quitting Islam. But as long as a Muslim has not quit Islam, he will be considered a Muslim, though he may be a wrongdoer Muslim (Fasiq). He may commit sins over sins and still be titled as Muslim, since he doesn't reject the rules of Allah and knows what he is doing is against the will of Allah, he is Muslim in belief although not being Muslim in deeds!
- In Christianity, salvation is through the Christ, what about in Islam?
About salvation, yes at least Shia Muslims believe in the concept. Shia scholars sometimes describe the issue by making an analogy between this life and traveling by bus. Suppose you are to move from your city to another city, if you are on the right bus you will reach the correct destination even though you may find your hands broken due to not fastening your seat belt. If you are on the wrong bus, you will never reach your correct destination, no matter how pleasant the trip! Simply being on the right bus is what really matters, not the inside decoration and facilities of the bus, or even its safety. Shia Muslims believe, based on Ahadith from the holy prophet and The Quran, that believing in Imam Ali's real rank is being on the right bus, and that the believer people may still suffer their wrong deeds but eventually (maybe even after thousands of years suffering in hell) they will survive; this is called intercession (Shafa'at).
This is somewhat similar to the concept of Salvation on Christianity but maybe not exactly the same. Christians believe that salvation is only through the Christ (if I am not wrong) and Shia Muslims believe the salvation is only through Imam Ali, peace be upon them both. However, I saw some Christians were talking like no non-Christian would ever reach salvation since none of them would accept Jesus as a son of God, and this is not what Shia Muslims say about those who are not Shia or even Muslim. What Shia Muslims believe is that no enemy of Imam Ali peace be upon him would ever enter the paradise, but not all the non-Muslims or non-Shia Muslims can be considered as enemy to him; everyone who likes good intentions and good deeds would like Imam Ali --peace be upon him-- very much, albeit if they know anything about him.
Of course, according to Shia Muslims' way of thinking, those who can gain more are those who know Imam Ali peace be upon him more, like him, and of course obey him. Obeying him is obeying the holy prophet and obeying the holy prophet is obeying Allah. Also I saw some Christians believing that when they confesses Jesus is God or His son it will suffice and there is no need to be good any more, at least not very seriously. According to the Shia way of believing, we should try our best not to commit any sin, be it major or minor in the sense classified by the religion. Imam Ali --peace be upon him-- has advised us not to look at the smallness of the sin, but to see the greatness of the one we are rebelling against by disobeying Him. Most of us cannot keep ourselves clean in practice; in the inside fight between desire and intellect, some examinations of Allah are indeed very difficult and Allah Himself has told us that human in general is created as weak:
Allah doth wish to lighten your (difficulties): For man was created
Weak (in flesh). [4:28]
Man is a creature of haste: soon (enough) will I show you My Signs;
then ye will not ask Me to hasten them! [21:37]
Truly man was created very impatient. [70:19]
and very loving of wealth:
إِنَّ الْإِنسَانَ لِرَبِّهِ لَكَنُودٌ \ وَإِنَّهُ عَلَىٰ ذَٰلِكَ
لَشَهِيدٌ \ وَإِنَّهُ لِحُبِّ الْخَيْرِ لَشَدِيدٌ
Truly man is, to his Lord, ungrateful; / And to that (fact) he bears
witness (by his deeds); / And violent is he in his love of wealth.
So that our need to Allah's forgiveness is not a trivial need for most of us. We would need the intercession of God's distinguished men's for salvation, and Allah wants us to stay on their door, to learn from them and obey them; for they are our patterns.