“Therefore, brethren, be even more diligent to make your call and election sure, for if you do these things you will never stumble.”
“Sure” means firm, permanent. We need to make our summons and selection sure for ourselves. “Sure” was a legal term in the first century for a guarantee. It was a legal guarantee obtained by a lawyer from the seller. This is a guarantee lest the third part claim the thing. Stability comes from our summons and selection. Our summons and selection is already present.
“Sure” stands emphatic in the Greek sentence. The Christian should stand firm on his feet with steadfast inner solidity. The Christian stands on solid ground when he legally validates the confirmation of the “sale” of his salvation. God will make good on His promises. God secures the payment of salvation. There is no “money back” guarantee because we will get the product.
We need to stabilize our salvation. It is not unstable from God’s standpoint. It is unstable from our standpoint.
We prove our salvation by fulfilling the injunctions of verses 5-7.
We cannot make our calling and election more sure than they already are but we can gain assurance that we belong to Him. God has already completed it but we can confirm the call in our mind by fulfilling the character qualities of verses 5-7.