When we translate the Greek conjunction γάρ as "for," we sometimes obscure the connection between verses. When Jesus says that the Pharisees wouldn't even get into the kingdom of God, how does that relate to his prior assertion that every iota and stroke of the pen found in the law will be fulfilled in him?
One of the more challenging conjunctions is γάρ. We tend to think it always means “for” in the sense of introducing a reason, but γάρ is much more fluid than that, and the English “for” isn’t.
Granted, the first definition in BDAG is, “marker of cause or reason, for.” This is its primary gloss, but only one among three.
Case in point is Matt 5:20. Beginning in v 17, Jesus is addressing the question he is expecting from the Pharisees, and that is his view of the Law. Jesus says that he did not come to abolish but to fulfill the law, every iota and stroke of it (v 18). Therefore (γάρ), whoever does not keep the least of the commandments is the least in the kingdom of heaven, and the one who does them is great.