Deuteronomy 23:21-23 speaks concerning worshipping God by keeping a vow to Him.
"When you vow a vow to Jehovah your God, you shall not delay in paying it; for Jehovah your God would certainly require it of you and it will become sin in you" (v. 21). To vow a vow to God and then not fulfill it, not carry it out, is sin. Once we make a vow to the Lord, we should fulfill it as soon as possible.
Some of the saints may want to make a vow to the Lord to serve Him full-time. Actually, God is "hungry" and "thirsty" for us to do this. He wants people to be for Him and to serve Him full-time. The worldly people, although created by God, have forgotten Him and have given Him up. They are busy in the world taking care of themselves and their own interests, leaving God hungry and thirsty, with hardly anyone willing to serve Him full-time. God is eager for the people created by Him to come back to Him. He is happy when some who have returned to Him promise to serve Him full-time. God is waiting for more of His people to do this.
"But if you refrain from vowing, it will not become sin in you" (v. 22). God is a God who respects our free will. This means that He does not compel us to offer ourselves to Him. God is thirsty for full-timers, but promising to serve Him in this way is a matter of our free will. If we do not make such a vow, God will not count it as sin that we do not vow. But if we vow a vow to the Lord, we must fulfill it. Otherwise, the failure to fulfill the vow will be counted as sin.
"What proceeds out of your lips you shall keep and do, as you have vowed to Jehovah your God voluntarily, which you have promised with your mouth" (v. 23). Those who made a vow voluntarily, promising God to do a certain thing, were required to keep and do what proceeded out of their lips. Today, those who promise to serve the Lord full-time should keep their promise and fulfill their vow.