In the object geojsonMarkerOptions1 you don't have access to the feature. It is an object used exclusively to assign a default style for your markers. To assign a different style for each marker you should provide a function to the style parameter of L.geoJSON.
Something like this should work:
var geojsonMarkerOptions1 = {
radius: 7,
fillColor: "grey",
color: "red",
weight: 1,
opacity: 1,
fillOpacity: 0.8
};
function getColor (feature) {
switch (feature.properties.Nab) {
case 'Katolici' : return {fillColor: "blue"};
case 'Protestanti' : return {fillColor: "red"};
}
};
L.geoJson(sidla, {
pointToLayer: function (feature, latlng) {
return L.circleMarker(latlng, geojsonMarkerOptions1);
},
style: getColor
}).addTo(map);