I take it as axiomatic that the only rational basis for any action is the expectation or hope that it will in some sense benefit the agent. If the agent does not expect to benefit, why should they act?
Many acts of kindness do not benefit the actor directly, but only indirectly - by tending (the agent hopes) to make society a generally kinder place and so making it a better environment for the sustenance of their own life.
The difference between virtuous actions and vicious actions is basically the scope of evaluation which the agent chooses when deliberating over what action they will take. A vicious action is taken on the basis of immediate and local advantage. A virtuous action is taken on the basis of wide-range and long-term benefit.
The problem with all this is that "in the long term we are all dead". So there is no rational basis for any individual to take into account the longest-term (or most global) impact of their acts. If physical death is the absolute end of one's consciousness than what happens after - or to things and people who are so remote from us that they have no effect on us - is of no consequence to the agent acting.
And yet one "feels" that the consequences of one's acts for "generations to come" and for "people the other side of the world" are somehow also important when evaluating the moral character of acts. Now it seems to me that either this feeling is mistaken and so the "kindness" which it tends to lead to is irrational, or else there is some basis for this feeling and so the kindness that is more globally motivated is rational.
It seems to me that what is required to make "global kindness" rational is the idea of "eternal life". This makes "global kindness" rational by removing the idea "in the end we all die", so that long term consequences (but in "the next world", I admit) become of paramount concern. Seriously believing that one is going to live for ever after death transforms one's attitude to the present life. This "seventy years" ceases to be at all important as such and is necessarily viewed as merely the prelude and preparation for eternity. Kindness is then entirely justified and made rational, on the basis that one must start practising the attitude that will be appropriate when living in community with a multitude of other souls for ever.
I suppose that it might be possible to conceive of eternal life without God - but I'd then question what was going to be the basis which supported such an existence. According the Judeo-Christian revelation, it is God who is the ultimate sustenance of all things; in this present world, in heaven and in the world to come.
If one does not believe that we humans have an eternal and transcendent aspect to our being, then I can see no possible means of underpinning ethics or of motivating kindness rationally. People have to appeal to arbitrary principles of "human dignity" or "the value of human life" or "justice" or "love" without giving any account as to WHY these (excellent principles) operate or have validity.
Clearly, many atheists do adopt excellent ethical stances. I am not in the business of saying that atheists are necessarily immoral and unkind and that only believers can be moral and kind! However, it seems to me that no atheist can rationally maintain a recognisable ethic, but only adopt it arbitrarily as an "ad hoc" and unjustified adjunct to the rest of their philosophy.