'Fast' in the Bible

But you, the [ones] holding fast to Yahweh your God, [are] all alive {today}.

Yes, if you diligently keep all this commandment that I [am] commanding you {to observe it}, by loving Yahweh your God {by walking in} all his ways and by holding fast to him,

You shall go after Yahweh your God, and him you shall revere, and his commandment you shall keep, and to his voice you shall listen, and him you shall serve, and to him you shall hold fast.