Yes. it is only sufficient to believe in oneself.
The word “atheist” is generally used to say that you don’t believe in God. But what is the idea of God in your mind? Where is he? Is he there in temple? is He is the stone? Or is he residing high up in the clouds? God is He or She or Both or None? People are not clear about the idea of God. Even the so-called believers don’t have right understanding about God. It is interesting to note that even atheists are considered and given a representation in Vedas (referred to as Charavakas).
Bhagavad Gita is not talking about God in temple or resides in heaven. Gita specifies that God is inside you and me and the purpose is to realize this God, the very nature of our Self, which is nothing but pure consciousness and happiness. Sanatana Dharma propounds this truth. That same God lives in everyone, you and me. It is not separate from you. Believe or not, without him we can’t even disbelieve him. So, it is only sufficient to believe in oneself.