Allah (swt) knows best who will go astray because He (swt) is the One Who turns away the hearts and eyes. He (swt) knows best who is rightly guided and who is misguided. The Prophet (saws) is only responsible to deliver the message and to remind, but he (saws) is not responsible for guidance. Alah (swt) says:

 إِنَّكَ لَا تَهْدِي مَنْ أَحْبَبْتَ وَلَٰكِنَّ اللَّهَ يَهْدِي مَن يَشَاءُ ۚ وَهُوَ أَعْلَمُ بِالْمُهْتَدِينَ

(Indeed, [O Muhammad], you do not guide whom you like, but Allah guides whom He wills. And He is most knowing of the [rightly] guided) (Al-Qasas:56)

      Another verse says:

 وَإِنَّكَ لَتَهْدِي إِلَىٰ صِرَاطٍ مُّسْتَقِيمٍ

(And indeed, [O Muhammad], you guide to a straight path) (Ash-Schura: 52)

The two above mentioned verses refer to two different meaning of guidance. The guidance of the Prophet (saws) is Directional Guidance; he (saws) guides all humanity to the straight path. Whereas the guidance of Allah (swt) is a guidance of support, which is placed in the heart. So, the Prophet Mohammad (saws) does not guide whomever he loves, nor can he (saws) enter their hearts to guide them.