Explain the Sartrean concept of existence preceding essence

The great 19th Century philosopher and writer Jean-Paul Sartre is most known now for his contributions and innovations in ontologocial philosophy which is now often dubbed 'Existentialism'. The concept which the question refers to is one Sartre posited most notably in his 1946 lecture titled 'L'existentialisme est un humanisme' (Existentialism is a Humanism). In this lecture (now printed as a book) Sartre writes 'Man first of all exists, encounters himself, surges up in the world - and defines himself later[...] To begin with [man] is nothing. He will not be anything until later, and then he will be what he makes of himself''. These ideas and thoughts would go on to be summed up with the simple mantra 'Existence precedes Essence'. This means that any perceptions of one's' self, the identity with which one associates, our mannerisms and thoughts and loves are only determined by the simple fact that we exist first. A good example is that of a laptop; if I were going to create a laptop I would certainly have a pretty good idea of what makes a laptop a laptop, If I wanted to I could list the characteristics of a laptop and when I have designed it in my head, when its essence is established then people may go ahead and try to make it exist. In this example, the essence of the laptop, preceded its existence. Though Sartre makes the switch when it comes to humans, nobody sits at a desk before they create a human and determines its essence, rather because all individuals are free-willed agents they are born and so exist and from that a person's essence is determined. Sartre's philosophy sparks many religious fires though, as his vision of existence leaves no room whatsoever for any Judaeo-Christian or Theist creation stories. If Genesis 1:27 says that 'God Created mankind in his own image, in the image of God he created them.' then unfortunately, Sartre deactivates that theory as mankind doesn't have an essence and QED he cannot create them 'in' any 'image'.