To understand the mainstream Christian justification for free will, you have to understand some Christian theology...
DISCLAIMER: If the following doesn't represent someone's theology, feel free to say so. I won't be offended.
NATURE OF GOD:
Most Christians believe that God has a trinitarian nature, meaning he is of three persons. That is, God (the Father) exists forever and outside of time/space. One aspect of that God is his perfect love. From that love naturally and eternally materializes another person to receive and then reciprocate that love, the Son. The love that is produced between father and son assumes an entirely new person, the Holy Spirit. All three persons eternally exist from the Father, self-contained, through perpetuity.
PURPOSE OF MAN:
With the exception of some Reformists, most Christians believe that God's purpose for creating man was to create a being with the capacity to fully offer love to God through a completely free act of will. The purpose of all existance, then, is to live like God in total, faithful self-giving and receptivity to God's love.
GIFT OF FREE WILL:
From this understanding of God's nature, it becomes necessary that everyone, regardless of past or circumstance, faces at least once in his/her life the utterly free choice to choose Him, or to deny Him. This is an ability that is granted by God, as a gift, though he does not need to do it. He chooses to restrain his control over the world in order to allow for free will. Logically speaking, any decision to love God that is not truly free, is not a true gift of self. The Father does not force or require the Son, nor does he force us. Unlike the Son's only nature of perfect love, however, we have a second nature, a fallen nature, that keeps us in constant struggle between our conflicting desires for worldy love versus God's love.
GOD'S INTERVENTION:
Instead of controlling our hearts, God
pursues our hearts. If we seek Him, He will respond directly by answering prayer or just by changing our lives dramatically for the better (or worse, if it's good for us). If he pursues us as individuals, it makes sense that he also pursues us as a race. The Exodus, the Passion and the Resurrection, were some ways he interceded to pursue the hearts of all of his children collectively.
FREEDOM VS LICENSE:
So we can acknowledge that other factors play into our decisions, but that God allows free decisions all the time, and that at some point in our decision-making process, God often (if not always) allows us a free choice. Those decisions are what we are judged on, so freedom it may be, but license to do anything we want, it is not.
One might think of this as a false freedom, since we get punished for choosing the "wrong path", but the ramifications of a free act of sin is to
choose to contradict our created nature and to
choose to move away from God. (This gets into more Catholic-centred philosophy here). Like asking God to leave the room. He respects your choice and will leave, though he doesn't stop pursuing you to come back. So, ultimately, your choices to distance yourself from Him, and to ignore his calling you back, will result in the natural end: to be completely distanced from Him - to be in hell.
CHOICE FOR NON-CHRISTIANS
Keep in mind that this decision to "choose God" also includes people who choose God's morality (which is written on all of our hearts, regardless of culture or upbringing). Many people have not even heard of Jesus, or were never given sufficient message of the gospels. These people can still follow the law of God, even if it comes in another less-recognizeable form. Catholics make no claim as to what God has in store for these people, so we simply assume the best and have faith in God's divine fairness.
Sorry, this explanation turned out to be longer than I had hoped. If you made it this far, let me know and I'll give you some blessings
Peace be with you,
