It can be tempting for us, as Christians, and in particular those who are privileged enough to have a Church background, to assume that everyone who opposes the Christian faith does so in spite of knowing it is true. This is why so many apologists bash unbelievers, and is even why some Christians oppose the very idea of apologetics.
In stark contrast to this belief is the sheer number of converts to Christianity whose testimonies include some form of argument that changed their mind. Nabeel Qureshi, for example, was an ex Muslim. When he was a Muslim, he was confident that Islam was true, right up until David Wood showed up and rocked his Islamic world. After he converted, he told David his previous thought process: Even if David made him 99% sure Christianity was true, he was still 100% convinced Islam was.
But we don't even need to go to modern converts to Christianity to know that ignorance can be genuine. Pay attention to Paul's testimony in his letter to Timothy: "And I thank Christ Jesus our Lord who has enabled me, because He counted me faithful, putting me into the ministry, although I was formerly a blasphemer, a persecutor, and an insolent man; but I obtained mercy because I did it ignorantly in unbelief. And the grace of our Lord was exceedingly abundant, with faith and love which are in Christ Jesus. This is a faithful saying and worthy of all acceptance, that Christ Jesus came into the world to save sinners, of whom I am chief. However, for this reason I obtained mercy, that in me first Jesus Christ might show all longsuffering, as a pattern to those who are going to believe on Him for everlasting life. Now to the King eternal, immortal, invisible, to God who alone is wise, be honor and glory forever and ever. Amen." (1 Timothy 1:12-17).
Why did Paul obtain mercy? "...because I did it ignorantly in unbelief." Paul's sin was, therefore, quite sincere. He wasn't rejecting Christianity because he knew it was true, but didn't want it to be. He rejected Christianity because He genuinely believed Christians were apostates, and God would continue to punish the Jews and keep their Roman oppressors over them if they did not cast out this blasphemy.
Now, if this is what happened with Saul the Persecutor, what are we to think of these atheist trolls, or these Muslim aggressors, these Left wing persecutors etc.? Well, first of all, we can still nevertheless say they are guilty of a grievous sin for their persecution of the Church. In fact, apart from Christ, no one can be saved. Every single person who leaves this world without coming to know Jesus as their Savior will perish, and spend eternity in the second death. The sincerity of their ignorance will, at best, mitigate their punishment (Luke 10:13-16, 12:47-48). You do not have to concede that a sin is "ok" in order to grant that it was committed in ignorance. In fact, the very charge of ignorance is a charge of error. It doesn't matter why they did wrong, they still did wrong.
However, just as we must concede that Paul, who testified and wrote under divine inspiration that He sinned in ignorance, did sin in ignorance, we can also afford to suggest that those who are not demonstrably lying are not, in fact, deliberately lying. That Muslim in the comments section who keeps scoffing "so you think 1 + 1 + 1 = 1"? He's obviously wrong about our faith, but he's scoffing at you because he sincerely believes Islam is the true religion, and you, as a Christian, believe a false and twisted polytheistic religion. You can afford to assume that.
Of course, even the most sincere of rejections can still be deliberate. It is possible to delude yourself into thinking what you want to think. There's a famous saying, "if serving the Lord is wrong, I don't want to be right". This is, of course, unreasonable; if serving the Lord is wrong, I want to stop serving the Lord. I want truth, so while I am sincerely a Christian, it's because I fully believe it is wrong not to serve the Lord. But we all have a certain aversion to hearing that which is against our beliefs. Thus, unbelievers would likewise say "if serving (insert false religion here) is wrong, I don't want to be right". If serving Allah, if serving Vishnu, if serving myself, is wrong, I don't want to be right. With this kind of attitude, they will never even hear that serving the Lord is right.
Therefore, just as you can afford to assume they are sincerely rejecting your Christian faith, you can afford to grant they are deliberately rejecting your Christian faith. Therefore, you do not necessarily have to share it with them either. "Do not give what is holy to the dogs; nor cast your pearls before swine, lest they trample them under their feet, and turn and tear you in pieces." (Matthew 7:6). "And whoever will not receive you nor hear your words, when you depart from that house or city, shake off the dust from your feet." (Matthew 10:14).
Ultimately, in our outreach, we must be like Christ. We must love everyone, even if we assume they will not respond in kind. We must assume they are sincere (unless shown otherwise) without losing our own sincerity. We must rebuke their pride, without becoming proud ourselves. And if it comes down to it, even if they persecute us to the point of death, we may pray for them "Father, forgive them, for they do not know what they do." (Luke 23:34, emphasis mine).
Ignorance is a common feature among the unbelieving world, who are undoubtedly deceived, blinded by Satan himself. They deserve condemnation, but the profound truth we, as Christians, must acknowledge is so do we. Jesus didn't hang on that cross because we are somehow more deserving than they are. He did it because we deserve to be hung on that cross. Our God saved us out of love, not duty, for justice would see us in the same fiery pit as the devil and his angels. Therefore, no matter how depraved, no matter how obnoxious, no matter how detestable an unbeliever may be, we must seek for them to receive the same mercy as Paul, the chief of sinners, who entered the Kingdom of Heaven to the sound of the joyful cheers of those whose blood was shed at his hand.