Why do people argue that the universe had to have a beginning because...?

well I'm not Christian, but my religion also believes in one God. When it comes to how this universe came to be what it is, we say only God can answer that, and it is one of those questions which can't be answered. Chritians, try to have an answer for everyhting no matter how ridiculous they may sound, but apparently they don't have answer for your question, except "God is outside time". no offense to christians though.