Church people often quote the familiar slogan that a sacrament is ‘an outward and visible sign of an inward and spiritual grace’, which is true enough as far as it goes. But there’s more to be said: in a sacrament, the Church teaches that God has promised that what is signified in the sacrament will be effected. This differentiates sacraments proper from things that are similar to sacraments (the sort of thing people call a sacrament when they want to indicate this or that communicates (to them, at least) that God’s grace is active in the world — vocal harmony, perhaps, or the practice of convivial visits with dear friends, or dawn over a mountain lake).
Moreover, though sacramental interaction will be effectual on the Godward side, people can still refuse grace by fraud or by repudiating (whether explicitly and deliberately, or implicitly by living in a way that repels the divine openness to swathing us with grace).