"I can do everything through him who gives me strength" (Philippians 4:13 NIV). We're not as strong as we think we are, but God is stronger than we think.
"I can do everything . . . " doesn't mean, 'Now that I'm a believer, I'm strong enough to do everything and anything for God. God gives you the strength you need for each day. Your ability to "do everything" is wholly dependent upon Him because your strength is dependent upon Him.
Strength comes from submission. The thing you do that may require the greatest strength is to submit yourself completely to God! But God is "working in you, giving you the desire to obey him and the power to do what pleases him" (Philippians 2:13 NLT).
