Praise be to Allah.
Allah says (interpretation of the meaning):
"Truly, the religion with Allah is Islam." [Aal ‘Imraan 3:19]
"And whoever seeks a religion other than Islam, it will never be accepted of him, and in the Hereafter he will be one of the losers." [Aal ‘Imraan 3:85]
The essence of Islam is submission to Allah alone, by worshipping Him alone with no partner or associate, and by obeying Him and His Messenger (sallallahu alaihi wa sallam). The basic principle of Islam is bearing witness that there is no god except Allah and that Muhammad is the Messenger of Allah. Every Muslim must practise the religion of Islam, devoting his worship sincerely to Allah and following the Prophet (sallallahu alaihi wa sallam). Whoever adheres to that until he dies will be one of the people of Paradise, but whoever does not enter Islam before he dies will be among the people of Hell. Whoever enters Islam then goes back (to kufr) is an apostate and a kaafir, and if he dies in that state of kufr (disbelief) he will be one of the people of Hell. But if he repents and comes back to Islam, and remains a Muslim until he dies, his apostasy will not affect him, and he will be one of the people of Paradise, even if he fell into apostasy more than once. But there is the fear that this repeated leaving Islam will cause him not to be guided to repentance. Allah says (interpretation of the meaning):
"Verily, those who believe, then disbelieve, then believe (again), and (again) disbelieve, and go on increasing in disbelief; Allah will not forgive them, nor guide them on the (right) way." [al-Nisaa’ 4:137]
So hasten to repent sincerely and adhere to Islam, fulfilling the obligations that Allah has enjoined on His slaves, the greatest of which are the five daily prayers. Avoid sin, and ask your Lord to make you steadfast in adhering to His religion. If you lose enthusiasm then seek the help of Allah and seek refuge with Allah from the Shaytaan. If you find that you are experiencing insinuating whispers which are causing you to doubt Islam or some of its principles, then turn away from them (those whispers), and seek refuge with Allah from the Shaytaan, and say "I believe in Allah and His Messenger."
You should recite Qur'an, and read books which will make you love Islam and encourage you to obey Allah, such as Riyaadh al-Saaliheen by Imaam al-Nawawi, and the Tafseer (Qur'anic commentary) of al-‘Allaamah ‘Abd al-Rahmaan al-Sa’di, Tayseer Kalaam al-Rahmaan fi Tasfeer Kalaam al-Mannaan.
Beware of books which make you doubt Islam and make sin attractive to you. Beware of bad company, for they are the devils among men. You should make friends with those who will help you to remain steadfast.
Beware of arguing about matters of religion, for this causes anxiety and confusion. Strive to worship Allah, for He guides those who strive for His sake to the straight path. Allah says (interpretation of the meaning):
"As for those who strive hard in Us (Our Cause), We will surely guide them to Our paths (i.e. Allah’s religion — Islamic Monotheism). And verily, Allah is with the Muhsinoon (good-doers)." [al-‘Ankaboot 29:69]