CNBC’s Jim Cramer on Monday said the benchmark S&P 500 could be at a pivotal moment this week.

“The charts, as interpreted by Mark Sebastian, suggest that we’re approaching a decisive moment here. If the S&P 500 can avoid a major decline this week, he’s feeling optimistic about the future — on the other hand, if it gets clobbered, he sees a heck of a lot more downside,” Cramer said.

The S&P fell 1.3% to 4,017.77 on Monday ahead of a packed week of earnings and a potential interest rate increase from the Federal Reserve.

To explain analysis from Sebastian, who is the founder of trading education company Option Pit, Cramer examined the daily chart of the S&P 500 going back to last January.

Sebastian notes that the S&P 500 hasn’t been able to break through its ceiling of resistance since a year ago despite multiple rallies last year, until Jan. 23 of this year, according to Cramer. The benchmark index has since stayed above the level for six consecutive trading sessions, he added.

He also compared the chart of the S&P over the last month to that of the CBOE Volatility Index, or the VIX, Wall Street’s fear gauge, to explain why the breakout is significant.

While the VIX and the S&P normally move in opposite directions, they’ve both moved up since Jan. 13, according to Cramer.

“When these two lines move in the same direction, it usually means you can’t trust the action in this, in the S&P … because while the market’s rising, the fear is rising, too,” he said.

Cramer added that Sebastian expects the market to see a retest of the new floor of support at the 200-day moving average.

“If that floor doesn’t hold, he wouldn’t be surprised to see the S&P plunging to a new low around 3,400,” he said, adding, “This is assuming something goes very wrong this week, maybe a much harsher than expected [Fed] meeting.”

For more analysis, watch Cramer’s full explanation below.