(Verse 14) - The word "render" is the same word used for sacrifice earlier. The idea is that this is the kind of sacrifice that is acceptable to God. This sacrifice included the physical sacrifices but went further than that and addressed the altitude that was needful for the sacrifice to be valid. It is not just the presentation of the bodies of animals that was acceptable to God. God was not seeking the blood of bulls and goats for Himself. Rather, He wants the sacrifice to be a genuine expression of the heart before God.

We must be sure to be clear that this is not dismissing the act of animal sacrifice. It was still, at this point, a necessary function of the faithful believer's life. Instead, we must see that what God is requiring here is a higher sacrifice from the heart of the giver. The point is that, without this "heart" aspect, the sacrifice is invalid.

This is underscored by the next phrase. God emphasizes that the vows are to be paid, but only when they are offered in conjunction with thanksgiving, the internal reality of a grateful heart, will that sacrifice be deemed valid.

The word ''vow" is the Hebrew word for an official vow or promise. The idea is that these physical offerings were to be an expression of the inner feeling and response to God's gracious activity in our lives. The point is that the vow is not fulfilled unless this is so.

This is why the mere offering of the physical component of the sacrifice was insufficient. These sacrifices had to be expressions of the inner gratitude, penitence, etc. that should, nay must be a part of all believer's experience with God.

(Verse 15) - There are two ways of viewing this phrase. First, we could see it as saying that this another portion of the offering itself. Some interpret the passage that way. It would be akin to the act of letting God be God.

Second, and more likely, this could be a reference to the result of verse 14 - that one can then call upon the Lord and expect a response. If we honor God with a proper response of praise and thanksgiving, we can be assured that God will, in turn, honor His promise to us and respond to our prayers and and deliver us.

Deliverance should be seen to refer to two things: present and future deliverance. We are delivered, certainly, from the present afflictions of life. This is assuredly so. However, the greater deliverance is yet future for us. The day we escape this world is the day of our greatest deliverance.

One other result is mentioned here: that of bringing glory to God. In reality, we can't truly bring God glory unless we meet the stipulation here. That is sobering! Good intentions be damned! God wants a grateful heart and willing spirit, not just observation of the letter of the Law! 

I need to be certain that the habits of my left are not just the rote observation of the physical  but that I am offering them with the grateful heart God desires of me.