No one can serve two masters; for either he will hate the one and love the other, or else he will be loyal to the one and despise the other. You cannot serve God and mammon.”
—Matthew 6:24 NKJV
Mammon here refers to money. We cannot serve God and money. But it does not mean that we will be poor or be in lack when we serve God. The servants of God mentioned in the Bible were never in lack.