Why does God advise us to avoid associating with unbelieving materialists?

The spiritual being who has received a human body, which is inherently more conducive to the realization of the Supreme Person, and who has dedicated themselves to the duties of loving and devoted service offered to Lord Krishna, becomes capable of realizing this, for they are the very embodiment of spiritual bliss.

Such a living being, wholly devoted to Krishna, the Supreme Person, is free from the influence of Maya, the energy of illusion. Even if they continue to reside in this world created by illusion, they are completely unaffected by it.

On the other hand, unbelieving materialistic souls, bound by Maya, are devoted only to their bellies and genitals. They are impure, and by associating with them, one will fall into the dark abyss of ignorance.

The intelligent person must renounce bad company and avoid associating with materialistic, unbelieving people, but instead be drawn to holy beings, great souls, and wise servants of God. Through their transcendental teachings, the Lord's devoted saints can break the false attachments of their minds. True saints are always free and consecrated to the Supreme Person.

Krishna, God, the Supreme Person teaches us:

Having attained this human form of life [the Lord is speaking of a soul that has been granted incarnation in a human body], which gives the opportunity to know me, and being situated in my devotional service, one can realize me, the reservoir of all pleasure and the Supreme Soul of all existence, residing in the heart of every living being. A person fixed in transcendental knowledge is liberated from conditioned life by renouncing their false identification with the attributes and modes of influence of material nature: virtue, passion, and ignorance. Considering these attributes and modes of influence of material nature as mere illusion, they (the incarnated soul) avoid becoming entangled with them. Because the attributes and modes of influence of material nature are simply not real, they do not accept them.