One of the Companions went near the Prophet Muhammad (saw) and said, “O Messenger of Allah! Give me a piece of advice about Islam so that I will not need to ask anybody else about it.” Then, the Prophet Muhammad (saw) said, “Say, ‘I have believed in Allah’, and be steadfast.” 
Human’s reason for existence is servitude to Allah, and the most important duty of human is to believe in the existence and oneness of Allah, and live steadfastly as required by such iman (faith). This is because once iman is absorbed in heart, it is manifested in feelings, thoughts, and behaviors. It leads one to the truth and truthfulness as well as honesty and steadfastness. Only then Muslims have a peaceful and honorable life. They become assured of troubles and sorrows both in this world and in the Hereafter. They attain taqwa and divine blessings.
Muslims on the true path do not make any concessions on righteousness. They do not bend their words and never tell a lie. They observe justice and do not gravitate towards injustice. They are well-intended and sincere, and are not ostentatious. They pay due diligence in their work and do it finely, and do not resort to deception. They, with their love and respect for the Creator, show mercy and compassion to the created beings and do not hurt any living being. In short, they live on the sirat al-mustaqim (the straight path). They see the consent of Allah greater than any gain or profit. They lead a life as aware that Allah (swt) sees and hears all things, concealed or open, and that He will call His servants to account.
I would like to conclude the khutbah with the following good news of our Lord (swt), “Indeed, those who have said, ‘Our Lord is Allah,’ and then remained on a right course – there will be no fear concerning them, nor will they grieve.” 
 Ibn Hanbal, III, 413.
 Ahqaf, 46/13.