Here is my understanding ---
All have received the measure of faith given by God (Rom 12:3 For I say, through the grace given unto me, to every man that is among you, not to think of himself more highly than he ought to think; but to think soberly, according as God hath dealt to every man the measure of faith).
We have all received the same measure. When all is said and done and we stand before God, no one can say he or she did not have what he/she needed in order to activate the measure given and come to saving faith in Messiah.
God also gave everyone a conscience. In Rom 2:15, the conscience is referred to as the work of the law written in the heart. Please note this is not the law written in the heart. That comes once a person is born again. But we all start out with the work of the law written in our hearts by which we are able to recognize good from evil.
Again, no one can stand before God and say he or she did not have what was needed in order to come to faith.
So, if all have the measure of faith and all have the work of the law written in the heart, why do some come to faith and some do not come to faith?
Romans 1:18 For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who hold the truth in unrighteousness
To "hold the truth in unrighteousness" is to suppress, restrain, or hinder the truth in unrighteousness. God has given us what we need in order to receive truth from Him. He gave us the measure of faith and He gave us the work of the law written in our hearts.
If/when we hear God's truth and we do not suppress, restrain, or hinder it, it flows to our heart where God can bring increase (1 Cor 3:7). The faith which God gave to each one of us in the same measure can grow stronger as we learn to trust in God.
However, if/when we hear God's truth and we suppress, restrain, or hinder it, it does not reach our heart where God can bring increase. We then remain weak in faith.
And reading further in Rom 1, we see what happens as we continuously suppress, restrain, and hinder:
Rom 1:21-23 – foolish heart becomes darkened. We then profess ourselves as wise but in reality we become fools and change to glory of God into an image (idolatry).
Rom 1:24-25 – because of our idolatrous ways, God releases us to uncleanness. We then dishonor our bodies by changing the truth of God into a lie by worshiping the creation more than the Creator.
Rom 1:26-27 – because we dishonor ourselves, God releases us to vile affections. We see Rom 1:26-27 going on in the US every day. And we are also seeing that some have received the recompence of their error which was due them.
Rom 1:28-32 – because we do not like to retain God in our knowledge, God releases us to a reprobate mind, which results in all of the activities we see in vss 28-32.
So, we have a choice to suppress, restrain, hinder God's Word or not suppress, restrain, hinder. That is the choice we are faced with at any given moment.