No one can harm the man who does not harm himself is actually a treatise by Saint John Chrysostom. The whole thing is here. It’s pretty long and I’m not going to get into the whole thing as you can read it yourself if you want to, but the concept is crystal clear alone from the title.
I will now conclude my discourse by repeating what I said at the beginning, that if any one be harmed and injured he certainly suffers this at his own hands, not at the hands of others even if there be countless multitudes injuring and insulting him: so that if he does not suffer this at his own hands, not all the creatures who inhabit the whole earth and sea if they combined to attack him would be able to hurt one who is vigilant and sober in the Lord. Let us then, I beseech you, be sober and vigilant at all times, and let us endure all painful things bravely that we may obtain those everlasting and pure blessings in Christ Jesus our Lord, to whom be glory and power, now and ever throughout all ages. Amen.
What harm can we do to ourselves except by choosing that which is sin?
Although we may be “harmed” in terms of what the world considers harm — we may be persecuted, we may lose it all, we can be poor, we can be impoverished, we can be ridiculed, we can be scorned, we can suffer, we can even suffer from those who call themselves Christians, we can suffer in our relationships and in our marriages — but in reality none of that is what harms us.
This is a true eternal perspective.