Thank you for requesting my help with this question.
These are very different approaches to interpersonal relational interactions.
In learning theory, personality is not a product of instinct or genetics but is something that is a result of experience throughout life. From the learning approach, personalities are shaped, reinforced and moved through learning from life experiences. Personality traits are stamped in via behaviors that produce desired results. And other aspects of personality are minimized through the same process.
In a learning view, interpersonal relationships are a series of shaped behaviors. You smile at me, and I smile back. I say I like cats and you smile further causing me to talk more about cats and magnifying those aspects of my personality that are socially related to conversations about animals. Basically, the interactions that I demonstrate to others as a part of my personality are adaptations to the needs of the environment. Basically, learning theory says that the environment dictates those aspects of personality that surface in our interactions.
Existentialism would be drastically different in it's explanation of how a personality is developed within interpersonal relationships. Contrary to learning theory, Existentialism is totally based on our freedom to choose our behaviors. We are viewed as alone in this world, but through our own development of meaning, we find ourselves. The word that explains this best is a German word called Eigenwelt which is self identity separate from others. This means that no single relationship can reinforce us into a personality set, rather, our personalities emerge according to how we see our meaning (value, purpose, existence) within a relationship. The relationship is always in the present and is not viewed with any degree of sameness so personality is in flux according to the relationships the person encounters.
Let me know if I can help in any other way,
One of the leading existential phenomenological schools in the US also happens to be a religious university. So no, this viewpoint does not require one to be a atheist. Rather, existentialism talks a great deal about man's freewill and our ability to choose our own path. In the Bible, God also gives people the ability choose their own paths, including whether or not to believe in Him.
On the other hand, what you have observed about Existentialism is also true. Many Existentialists are atheists or agnostics. However, so are the majority of Humanists, psychoanalysts, and many behaviorists. Indeed, the entire field is very secular and at times a belief in God is difficult to integrate. Even though this so, major exceptions exist. There are entire Universities whose focus on psychology is devoted to Man's need to worship and recognize God as creator and Lord.
So, although freewill and a search for self-meaning is a part of Existentialism, it is not always "God absent"...
Thank you for the request on your question today, June 4th. However, it is in the Homework category and I am not able to answer in that category. If you want to resubmit your question in the mental health category, I will be available this afternoon to answer it for you.