Is it necessary to use activation functions in neural networks?

Is it necessary to use activation functions in neural networks?

The activation function is a node that is put at the end of or in between Neural Networks . They help to decide if the neuron would fire or not. “The activation function is the non linear transformation that we do over the input signal. This transformed output is then sent to the next layer of neurons as input.”
ref: https://medium.com/dataseries/basic-overview-of-convolutional-neural-network-cnn-4fcc7dbb4f17