#SunnahFriday - Greatest Sin
The biggest sin is to attribute partners with Allah (SWT) by seeking others means out of a certain condition or predicament. This my include simple things such as asking someone to remove a burden or extremes such as speaking before graves asking the dead for help.
"And who is more astray than one who invokes besides Allah, such as will not answer him till the Day of Resurrection, and who are unaware of their invocations to them? And when mankind are gathered (on the Day of Judgement), they (false deities) will become enemies for them and will deny their worshipping." - Q46:5-6
There are many forms of disbelief and some are right in front of our eyes; hiding in plain sight. There are many people who have that lucky shirt they must wear to interviews or a bracelet that they believe protects them from evil. The moment we cross that fine line of believing the item actually does what we believe it does, we have committed a grave sin.
The Prophet Muhammad (SAW) said: "Whoever wears an amulet, may Allah not fulfill his need" - Ahmad
Disbelief can also manifest by swearing by anyone or object other than Allah such as a person's honor, his status or even parents. Anyone who swears by anything other than Allah has committed shirk. And the only remedy of accidentally saying such is by repeating la ilaha illa Allah - there is no god but Allah.
"Whoever swears by al-lat and al-uzza (idols worshipped before Islam) should say there is no god worthy of worship but Allah." - Bukhari
"Whoever visits a soothsayer and believes in his words, has denied the Revelation given to me" - Ahmad
Reading and believing in horoscopes is taken as a pastime, with many people actually ascribing behaviors and personalities to it. Belief in horoscopes, fortune tellers and use of magic are all prohibited to Muslims. We must do our best to avoid such acts and stay clear of them.
"Whoever goes to a fortune-teller and asks him about something and believes him, his prayers will not be accepted for worthy days" - Muslim
"What lies between a man and disbelief is the abandonment of prayer" - Muslim