Christians may say that because the person you come in touch with is made in the image of God, you must love that person as yourself. If the idea is that the person is somehow a picture of God and God is so great that we must honour a person just because of that, isn't that really using God as a crutch? You think the person is worthless and need to bring God into it to respect them. To argue that your dead spouse's picture is to be honoured as much as her for it is her image is insane and does not follow. Usually the image thing is said to refer to how we have the same power as God to bond with others and form relationships as he does. So now we have, "God made us for relationships therefore we must care for each other." That is like saying you must never use a walking stick as a lever for it is made for walking.
Maybe the best way to look at it is that in the background, the image of God thing is saying there is a God and he has put this person beside you therefore you should help and do only good for that person. No room for indifference or cruelty here. In fact the image of God teaching looks totally vacuous without this arrangement doctrine. So in essence, you are respecting God through respecting his plan which put this person who you must treat as a friend beside you. It is going to backfire for it is too demanding and only makes cherry-pickers. Literally nobody is that neighbourly. With issues such as racism, it is going to make you say the right things but as for your actions, they will fail too much. They will show the real you.
Some run to the children of God doctrine for help here.
The Bible regards all as children of God in the sense that God made them but holds that some who take on the duty to serve God in the way he requires, thought to be water baptism, are the proper children. The Catholic Catechism says we are adopted children of God only if properly baptised. That explains why the commandment is, “Love your neighbour as yourself” and not, “Love your brother or sister as yourself.”
It really amounts to saying we should help each other and try to treat the other person as another self. It does not tell you to make the effort to get rid of feelings of indifference or dislike. That is its flaw for helping a person is no good when their mental health will fall to pieces over nobody liking them. In fact helping them won't really help in many ways.
Atheists however say we came from animals and so we must treat each other well no matter what. Critics say that does not follow.
But if God has to give you the reason to care for your neighbour, it is up to him how he gives it. He can decree that because we are animals and thus vulnerable that is why we should care.
Position 1 is that our reflecting God should automatically mean that a person should be loved. That is not clear. If God commands that does not mean that God has to command. If God is in control to look after people then our attitude shouldn't count.
Position 2 is that being vulnerable calls for somebody to help so if I am an animal I have needs and they need to be met. If there is a God then he is deliberately standing by to let you be harmed or helped. Our being vulnerable should suggest that a being that can help must help and would suggest that there is either no God or no caring God.
Perhaps neither of the two positions follow. We could still go for Position 2 by a process of elimination for it is still better than the first. It has something about it.
No animal exists unless something cares enough, the mother perhaps? So being an animal means caring. So the atheist argument is absolutely fine. Its rival is not truly as respectful of God as it seems for it tells him how he is to affirm neighbourly love.
Another complaint is that atheists are upholding the law of God to serve neighbour in that respect but rejecting the lawgiver. That is said to be shallow and unfair. So they are trying to argue that though a person can be moral without faith, that person is inconsistent. They compare it to, "We just know we should not drive through red traffic lights. It does not matter if nobody made that law or not." That is a trick argument. It is true that somebody made the traffic law but there was a sense in which it made itself. You need a warning and can't just drive on. They want you to think, "I in fact have no respect for law at all when I don't even acknowledge that there are persons authorised to make the law." That is rubbish. You want and need the rule and its just happens that somebody authorised it. That somebody does not matter.
Love your neighbour as yourself is largely about treating most of the human race as follows. "We are the benefactors and children of God and they are not yet we help them". That is patronising and leads to division down the line. If we come from a God is it not clear how serving others is necessarily an obligation. Remember if God is bigger than our sins and errors then he is doing the looking after and it is no big deal if somebody hurts others. We could say there is more good in the evil than we will ever notice. We could say that the evil will fizzle out and destroy itself allowing good to rush in. Religion already says we don't even notice much of the good that prayer does. That only leads too many to be complacent and makes the diligent socially minded person lazy eventually. If they were persons of faith they helped others so well in spite of their faith and that is that.
We see then that if it is not clear how we can care for each other if we are animals it is also as clear as mud if there is a God too and we are in his image. But in fact there is something about our vulnerability that prompts helpful action. We don't need to dissect that or overthink. Why just not leave it alone? We are ignoring how Hume declared that an is does not give rise to an ought for the purpose of this argument.