People disillusioned with mysterious, mythical, irrational pagan practices like idol worship, incarnation, reincarnation of God, gods in human forms with wives & offspring and all other human limitations; get attracted towards Atheism due to its outward pragmatic and scientific appeal. Pragmatic Christians following the command of Jesus Christ that: “Why don't you judge for yourselves what is right?”(Luke;12:57), once undertake serious study of the Bible [especially the Gospels] are disappointed [like their learned predecessors, Arius (336 C.E) & Michael Servetus(1553 CE)].
Humanism is the retort by desperate pragmatic Christian scholars [Unitarian Universalists], to the mythical, unscientific and illogical traditional Christian Doctrines, however they have gone far away from one extreme to another. Islam provides a balanced, scientific and rational solution, while maintaining the unity and existence of God, the Creator, Cherisher and Sustainer of universe, who has not created any thing without a purpose. God Says: “Our creation of the heavens and the earth and all that lies between them is not a game. Had We meant to make it a play ground, We would have done it by Ourself, without giving you discretion to do right and wrong, had We ever done so. Nay, but [by the very act of creation] We hurl the truth against falsehood, and it crushes the latter: and lo! It withers away. But woe unto you for all your [attempts at] defining [God]”(Qur’an;21:16-18).
Humanists intend to serve the humanity, but with atheist concepts, devoid of accountability upon resurrection, it will remain a distant dream because people will lack inner motivation. People who suffer in this life being poor, or due to birth defects, incurable diseases, or fall prey to calamites [both natural or man made] can never be happy unless they hope to be compensated in hereafter. No system can work if it is without checks and balances, which do put some limits on the freedom. Islam is the only faith which provides guidance for successful way of life for both worlds, not restricted to rituals and dogmas but actions and performance of good deeds, while avoiding evils.
Finally just for the sake of argument. if hypothetically one may consider the Humanism viz Monotheism, and grant probability of either being right or wrong as 50:50 [irrespective of strength of arguments of monotheist]. If 50% probability of Humanists being false materialise the Humanists are big losers because they have to suffer in hell fire for ever being unbelievers, while the true monotheists believers will ultimately be in a state of promised pleasure. In other situation assuming, if 50% probability of Humanists being ‘true’ materializes, the monotheists have nothing to loose, they had lived as good human in this world, performed good deeds to please their God & fellow human by establishing an equitable society. Hence believers are at win-win situation. http://quran-pedia.blogspot.com/2011/08/humanism-atheism.html