My soul shall be satisfied as [with] marrow and fatness
When he should return to the house of the Lord, and partake of the provisions of it, called the fatness of his house, (See Gill on Psalms 36:8). The phrase denotes the abundance of spiritual refreshment and delight in the word and ordinances, and the great satisfaction had in them; and may have some regard to benefits arising from prayer, as well as other ordinances. Fat was not to be eaten under the legal dispensation, and therefore not to be literally taken; but in the typical and spiritual sense which David understood, and therefore respects that, or otherwise he would speak contrary to the law of God: he refers to those spiritual good things which they typified, and give spiritual pleasure and satisfaction;
and my mouth shall praise [thee] with joyful lips;
such a full meal, such a rich entertainment, calls for abundant thankfulness; which is here signified by the mouth praising the Lord, and doing this with lips of shouting, expressions of joy, songs of praise, jubilee songs. The allusion is to the use of music and singing at festivals; see ( Isaiah 5:12 ) .