For your own self-designation, begin with the broadest most relevant category and call yourself a Humanist.
This classification encompasses all others. After the word Humanist you may add your preferred subset label: Humanist Muslim, Humanist Christian, Humanist Buddhist, Humanist Atheist, Humanist Agnostic. It is important that these subset terms do not precede the word Humanist. We are Humanists first, and what we are after that is secondary or tertiary or even further down the line.
We are Humanists first because we are human infants first, insusceptible of further branding at that time. No infant is Muslim or Christian or Atheist or Conservative or Liberal or even American or Dutch or Egyptian or any of the like. An infant is simply human, inducted by dint of that condition into a decades-long participation in 'basic human goods,' chief of which are friendship, play, learning, skillful performance, and the rearing of children.
We are Humanists first because Humanism is easiest to believe. There are no fabulist doctrines to embrace. No winged ponies. No uncertain nativities. No staggering saintly pedigrees. No post-possessed recuperations. No impracticable moral embargoes. No otherworldly opinions on textiles that drape the body. No deistic dietary whims. No lurching angel trumpeting doom. No underworld chamber brutalizing dissent. Humanism says simply that human ingenuity is the source of goodness and therefore a source of delight. Easy assent.
We are Humanists first because, apart from the rough generosity nature bestows, humans mold a malleable nature on behalf of human flourishing, creating innumerable gracious alterations to the natural world, from a road to a bridge to a house to a knife to a plate to a toy to a pipe to a balm to a bed to a flute to a lute to a wending nighttime story in florid prose, and a near infinity more.
We are Humanists first because with human tools we attempt an anatomy of human destructiveness to better ourselves and shed our vices. With human tools we devise the ethics of urgency to rapidly contain a rapacity that harms the elements and the animals.
We are Humanists first because we cannot be reduced to anything smaller than the human, and we cannot be elevated to anything larger. Even the posthuman and the transhuman are but species of the human--from which, for now, all the metrics of morality emerge and extend to bonobo and cyborg.
For what you call yourself, begin with the broadest most relevant term. When someone asks 'What are you?" say 'I am a Humanist.' Search and see what the word has meant. And as you understand it, receive it first and foremost, after which, if you must, you can array yourself with any of the other supplementary hues on offer. But start with Humanism.
for further musings see uponreligion.com