The General Choosing Volunteers
God knows everything before it happens, which is foreknowledge. That means he knows ahead of time who will call on Him to be saved and who won't. First Peter 1:1-2 says we are "chosen according to the foreknowledge of God the Father."
Even though God does the choosing, we are given the opportunity to ask Him to choose us. It's like the army general who asked his soldiers, "Whoever would like to volunteer to go on a dangerous mission with me, please step forward." About one-third of the soldiers stepped forward of their own free will, separating themselves from the other soldiers.
The general then went up to each soldier who stepped forward and said, "I choose you and you and you...." He chose everyone who stepped forward, but he didn't choose anyone who didn't volunteer. He gave them their free will, but he did the choosing.
In the same way, God allows us to have free wills, but He chooses everyone who wants to love Him. (Kent Crockett, The 911 Handbook,