God can intervene in situations when we pray to Him in faith such as bringing conviction of sin on the heart and mind of a sinner and putting the impulse in them to follow Him, but that does not negate free will. He has the power to soften the sinner's heart so that they call on Him to be saved. Also, the fact that God gave free will means He cannot be blamed for evil because it is man that composes evil. He can permit evil and also intervene for example the account of the flood in the days of Noah.
One can also question the purpose of evangelizing if God has predetermined who are His or God does not interfere with one's free will. Well Jesus commanded that we preach the gospel to the lost and the bible says "how shall they believe in him of whom they have not heard? and how shall they hear without a preacher?". So clearly, we are to preach the gospel because faith comes by hearing the word of God and His word, which is sharper as twoedged sword, has power to convict the sinner of their guilty state then they will repent and believe to be saved and God's name alone will be glorified.
Rom 10:1 Brethren, my heart's desire and prayer to God for Israel is, that they might be saved.
My point is that God can touch the heart even the hardest heart through His word and Spirit, so it is important that we faithfully pray for lost souls to come to the knowledge of the truth.