The verse "Now try the Lord Almighty, and you will never know anything" (Judith 8:13) is a statement by Judith challenging the people of Bethulia's plan to test God by setting a deadline for his intervention. Let's break down its meaning:

Context: The Israelites in Bethulia are under siege by the Assyrian army led by Holofernes. Desperate, the town leaders propose an ultimatum: if God doesn't deliver them within five days, they will surrender.
Judith's Rebuke: Judith, a devout and intelligent widow, strongly opposes this plan. She sees it as a dangerous and arrogant attempt to manipulate God.
"Now try the Lord Almighty": This is sarcastic. Judith is using the word "try" in the sense of "put to the test" or "challenge."
"And you will never know anything": This is the key to understanding the verse. Judith argues that by trying to force God's hand, they are demonstrating a lack of faith and understanding. She believes that if they try to control God's actions by setting conditions and deadlines, they will:

Fail to understand God's ways: God's timing and methods are often beyond human comprehension. Trying to dictate them will only lead to confusion and disappointment.
Lose the opportunity to experience God's power: By limiting God with their demands, they may miss the chance to witness his deliverance in his own way and time.
Risk losing their faith: If God doesn't act according to their timetable (which is likely), their faith could be shaken or destroyed.

In essence, Judith is saying that testing God in this way is foolish and presumptuous. It reveals a lack of trust in God's wisdom and power and will ultimately lead to ignorance and spiritual ruin.

She believes that instead of trying to manipulate God, they should humbly trust in his promises and allow him to work according to his own divine plan. This verse is a call for faith and trust in God's providence, rather than attempting to control or dictate his actions.