It is to set up others as partners with Allaah, and equating them with the Lord of the Worlds. As well as loving someone as one should love Allaah, and fearing someone as one should fear Allaah, or relying upon him, calling upon him, dreading him, hoping in him and having awe of him, trusting in him, or obeying him in disobedience to Allaah, or following him in what displeases Allaah etc.
Allaah, the Most High, says, “Verily! Allaah forgives not setting up partners with Him, but He forgives whom He wills sins other than that, and whoever sets up partners with Allaah in worship, he has indeed invented a tremendous sin.” (an-Nisaa’: 48)
Allaah, the Most High, says, “And whoever sets up partners in worship with Allaah, has indeed strayed far away.” (an-Nisaa’: 116)
Allaah, the Most High, says, “Verily, whosoever sets up partners with Allaah, then Allaah has forbidden Paradise to him, and the Fire will be his abode.” (al-Maa’idah: 72)
Allaah, the Most High, says, “...and whoever assigns partners to Allaah, it is as if he had fallen from the sky, and the birds had snatched him, or the wind had thrown him to a far off place.” (al-Hajj: 4)
The Prophet (sallallaahualaihi wasallam) said, “The right of Allaah upon His slaves is that they should worship Him alone and not associate anything with Him, and the right of the slaves upon Allaah is that He will not punish the one who does not associate anything with Him.”
And with regards to expulsion from the deen, there is no difference between those who openly declare their shirk like the disbelievers of Quraysh and those who hide it, like the hypocrites who declare Islaam but hide their disbelief.
Allaah, the Most High, says, “Verily, the hypocrites will be in the lowest depth of the Fire; no helper will you find for them. Except those who repent, do righteous good deeds, hold fast to Allaah, and purify their religion for Allaah, then they will be with the believers.” (an-Nisaa’: 145-146)